backend updates

pd-3ds:
Add support for RGB565 textures
pd-desktop:
Add support for A8 textures
Add glfw callback chain to not break other libs using the same callback

backends/desktop/include/pd-desktop/bknd-gfx.hpp

@@ -57,6 +57,7 @@ class GfxGL2 : public GfxDriver {
   GLuint Shader;
   GLuint pLocProjection;
   GLuint pLocTex;
+  GLuint pLocAlfa;
   GLuint VBO, IBO;
 };
 }  // namespace PD