refactor font code into classes

engine/app.h

@@ -4,23 +4,16 @@
 #include "shape.h"
 #include "texture.h"
 #include "layout.h"
+#include "font.h"
 
 class App
 {
     Sampler sampler;
     Texture2D tex;
     LayoutManager layout;
-    stbtt_fontinfo font;
-    Texture2D font_tex;
-    std::vector<stbtt_bakedchar> chars;
     Plane plane;
-    GLuint font_array;
-    int font_array_count = 0;
-    GLuint font_buffers[2];
-    const int w = 512;
-    const int h = 512;
-    const int num_chars = 96;
-    const int start_char = 32;
+    Font font;
+    TextMesh text;
 public:
     static App I;
     float width;