Batch all TTF glyph of a font in a single atlas

[Gallasko]

Right now a new texture is generated for each unique text written using a TTF Font, making the number of texture and drawcall increase really fast when writting a lot of text

OpenGLTexture fontTexture;

TTF_Font * font = TTF_OpenFont(fontPath.c_str(), textSize);

if (not font)
{
    LOG_ERROR(DOM, "Font could not be loaded: " << fontPath << ", error: " << TTF_GetError());

    return fontTexture;
}

SDL_Color color = {255, 255, 255, 255};

SDL_Surface * sFont = TTF_RenderText_Blended(font, text.c_str(), color);

if (not sFont)
{
    LOG_ERROR(DOM, "Font surface could not be generated" << ", error: " << TTF_GetError());

    TTF_CloseFont(font);

    return fontTexture;
}

auto w = nextpoweroftwo(sFont->w);
auto h = nextpoweroftwo(sFont->h);

auto intermediary = SDL_CreateRGBSurface(0, w, h, 32, 
        0x00ff0000, 0x0000ff00, 0x000000ff, 0xff000000);

SDL_BlitSurface(sFont, 0, intermediary, 0);

GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, intermediary->pixels);

sizeMap[textureName] = TTFSize{w, h};

fontTexture.id = texture;
fontTexture.transparent = false;

SDL_FreeSurface(sFont);
SDL_FreeSurface(intermediary);
TTF_CloseFont(font);

return fontTexture;