add progress bar to lightmap rendering

src/font.h

@@ -16,12 +16,15 @@ public:
     const int h = 512;
     const int num_chars = 96;
     const int start_char = 32;
+    // {mix, max}
+    glm::vec4 bounds{ 0 };
     int size = 0;
     stbtt_fontinfo font;
     Texture2D font_tex;
     std::vector<stbtt_bakedchar> chars;
     
     bool load(const char* ttf, int sz);
+    void calc_bounds();
 };
 
 class FontManager