GPUOffScreen: Remove the sample parameter

This is because the DRW module is no longer compatible with drawing using
MSAA.

This also change the Python API.

source/blender/editors/render/render_opengl.c

@@ -766,7 +766,7 @@ static bool screen_opengl_render_init(bContext *C, wmOperator *op)
 
   /* corrects render size with actual size, not every card supports non-power-of-two dimensions */
   DRW_opengl_context_enable(); /* Offscreen creation needs to be done in DRW context. */
-  ofs = GPU_offscreen_create(sizex, sizey, 0, true, true, err_out);
+  ofs = GPU_offscreen_create(sizex, sizey, true, true, err_out);
   DRW_opengl_context_disable();
 
   if (!ofs) {