Implement optimizations for 2D text rendering (#23)

The following patches are included:

## J2D: Rewrite text rendering

This patch rewrites text rendering for `J2D::DrawString` to now construct
a texture atlas for all ASCII-range glyphs in the FT font face, instead
of constructing a texture for every glyph.

This improves text rendering performance for several reasons:

1. Binding textures is relatively expensive as the GPU is required to do
   a context switch for internal data like texture parameters, and also
   cannot optimize for accesses to the same texture across draw calls.
   This patch removes the need to call `glBindTexture` more than once per
   call to `J2D::DrawString`.
2. As a consequence of the above, all glyphs for a given string can now
   be rendered in a single call to `glDrawArrays`. This is done by storing
   the cached texture coordinates on `CachedGlyph` and constructing a full
   array of vertices and texture coordinates for the entire string at
   once, resulting in only /one/ set of client-to-device attribute
   uploads and only one draw call, instead of being required to upload
   attribute data for each glyph separately.

## FontCache: Use map for efficient glyph lookup

This patch updates `CachedFont` to now use an `std::map` for cached glyphs,
instead of an `std::vector`. `std::map` allows O(log n) lookup, whereas
`std::vector` only allows O(n) lookup.

Note: `std::unordered_map` technically has better lookup complexity here,
with amortized O(1) lookup. However, hashmaps have a higher inherent
overhead than red-black trees so this would only be viable when going
above around 1000 entries, which should never happen here for ASCII
glyphs.

Co-authored-by: Ori Sky Farrell <ori.sky.farrell+git@gmail.com>
Reviewed-on: #23
Co-authored-by: ori_sky <redacted@ori.mx>
Co-committed-by: ori_sky <redacted@ori.mx>

src/FontCache.cpp

@@ -7,12 +7,11 @@ char CachedGlyph::getCharacter() {
     return character;
 }
 
-const GLuint* CachedGlyph::getTexture() {
-    return &texture;
+const std::array<GLfloat, 12> CachedGlyph::getTexCoords() const {
+    return texcoords;
 }
 
-CachedGlyph::CachedGlyph(GLuint texture_id, char c, float x2offset, float y2offset, float w, float h, float advanceX, float advanceY) {
-    texture = texture_id;
+CachedGlyph::CachedGlyph(char c, std::array<GLfloat, 12> texcoords, float x2offset, float y2offset, float w, float h, float advanceX, float advanceY) {
     character = c;
     this->x2offset = x2offset;
     this->y2offset = y2offset;
@@ -20,13 +19,14 @@ CachedGlyph::CachedGlyph(GLuint texture_id, char c, float x2offset, float y2offs
     this->h = h;
     this->advanceX = advanceX;
     this->advanceY = advanceY;
+    this->texcoords = texcoords;
 }
 
 //TODO
 //Because most things shown would be english characters. We can cut down on the iteration time significantly
 //by putting each english character at the beginning of the list in order of how often they usually occur in text.
 void JGL::CachedFont::appendGlyph(JGL::CachedGlyph* glyph) {
-    glyphs.push_back(glyph);
+    glyphs.emplace(glyph->getCharacter(), glyph);
 }
 
 unsigned int JGL::CachedFont::getFontSize() {
@@ -38,63 +38,48 @@ unsigned int JGL::CachedFont::getFontIndex() {
 }
 
 CachedGlyph* JGL::CachedFont::getGlyph(char c) {
-    for (const auto& g : glyphs)
-        if (c == g->getCharacter())
-            return g;
+    auto it = glyphs.find(c);
+    if (it != glyphs.end())
+        return it->second;
     return nullptr;
 }
 
-CachedFont::CachedFont(unsigned int font_size, unsigned int font_index) {
+CachedFont::CachedFont(GLuint texture_id, GLsizei texture_width, GLsizei texture_height, unsigned int font_size, unsigned int font_index) {
+    this->texture = texture_id;
+    this->texture_width = texture_width;
+    this->texture_height = texture_height;
     this->font_size = font_size;
     this->font_index = font_index;
 }
 
-void CachedFont::eraseGlyph(CachedGlyph* glyph) {
-    if (glyph == nullptr)
-        return;
-
-    for (int i = 0; i < glyphs.size(); i++)
-        if (glyphs[i] == glyph)
-            glDeleteTextures(1, glyphs[i]->getTexture()),
-            delete glyphs[i],
-            glyphs.erase(glyphs.begin() + i);
-}
-
-void CachedFont::eraseGlyph(char c) {
-    for (int i = 0; i < glyphs.size(); i++)
-        if (glyphs[i]->getCharacter() == c)
-            glDeleteTextures(1, glyphs[i]->getTexture()),
-            delete glyphs[i],
-            glyphs.erase(glyphs.begin() + i);
-}
-
-void CachedFont::eraseGlyph(GLuint texture_id) {
-    for (int i = 0; i < glyphs.size(); i++)
-        if (glyphs[i]->getTexture() == &texture_id)
-            glDeleteTextures(1, glyphs[i]->getTexture()),
-            delete glyphs[i],
-            glyphs.erase(glyphs.begin() + i);
-}
-
-std::vector<CachedGlyph*> CachedFont::getGlyphs() {
+std::map<char, CachedGlyph*> CachedFont::getGlyphs() {
     return glyphs;
 }
 
+const GLuint* CachedFont::getTexture() {
+    return &texture;
+}
+
+const GLsizei CachedFont::getTextureWidth() const {
+    return texture_width;
+}
+
+const GLsizei CachedFont::getTextureHeight() const {
+    return texture_height;
+}
+
 void FontCache::appendFont(CachedFont* font) {
     cachedFonts.push_back(font);
 }
 
-void FontCache::newFont(unsigned int font_size, unsigned int font_index) {
-    auto* font = new CachedFont(font_size, font_index);
+void FontCache::newFont(GLuint texture_id, GLsizei texture_width, GLsizei texture_height, unsigned int font_size, unsigned int font_index) {
+    auto* font = new CachedFont(texture_id, texture_width, texture_height, font_size, font_index);
     cachedFonts.push_back(font);
 }
 
 void FontCache::eraseFont(CachedFont* font) {
     for (int i = 0; i < cachedFonts.size(); i++) {
         if (cachedFonts[i] == font) {
-            for (auto& g: cachedFonts[i]->getGlyphs())
-                cachedFonts[i]->eraseGlyph(g);
-
             delete cachedFonts[i];
             cachedFonts.erase(cachedFonts.begin() + i);
         }