Added note on transparent framebuffer w/o X11 on RPI

src/core.c

@@ -3230,6 +3230,7 @@ static bool InitGraphicsDevice(int width, int height)
 
     VC_DISPMANX_ALPHA_T alpha;
     alpha.flags = DISPMANX_FLAGS_ALPHA_FIXED_ALL_PIXELS;
+    //alpha.flags = DISPMANX_FLAGS_ALPHA_FROM_SOURCE;       // TODO: Allow transparent framebuffer! -> FLAG_WINDOW_TRANSPARENT
     alpha.opacity = 255;    // Set transparency level for framebuffer, requires EGLAttrib: EGL_TRANSPARENT_TYPE
     alpha.mask = 0;