Request 24 bit depth buffer, to prevent z-fighting.

This should fix issue #6 with Intel and AMD gpus.

src/sdl_main.cpp

@@ -75,6 +75,9 @@ int main(int argc, char *argv[])
     // Allow us to use automatic linear->sRGB conversion.
     SDL_GL_SetAttribute(SDL_GL_FRAMEBUFFER_SRGB_CAPABLE, 1);
 
+    // increase depth buffer size
+    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
+
     uint32_t windowFlags = SDL_WINDOW_OPENGL;
     if (app.IsFullscreen())
     {