throw error when user tries to use Canvas background without sample buffer

drivers/gles3/rasterizer_scene_gles3.cpp

@@ -4335,6 +4335,10 @@ void RasterizerSceneGLES3::render_scene(const Transform &p_cam_transform, const
 			if (storage->frame.current_rt->buffers.active) {
 				current_fbo = storage->frame.current_rt->buffers.fbo;
 			} else {
+				if (storage->frame.current_rt->effects.mip_maps[0].sizes.size() == 0) {
+					ERR_PRINT_ONCE("Can't use canvas background mode in a render target configured without sampling");
+					return;
+				}
 				current_fbo = storage->frame.current_rt->effects.mip_maps[0].sizes[0].fbo;
 			}