drm: Don't use the cursor overlay if the scale doesn't match

If the scale for the cursor surface doesn't match that of the output
then we shouldn't use the cursor overlay because otherwise it will be
drawn at the wrong size. This problem is particularly noticable with
multiple pointers because it randomly alternates between drawing one
cursor or the other at a larger size depending on which one gets put
in the cursor overlay.

src/compositor-drm.c

@@ -953,12 +953,15 @@ drm_output_prepare_cursor_view(struct weston_output *output_base,
 {
 	struct drm_compositor *c =
 		(struct drm_compositor *) output_base->compositor;
+	struct weston_buffer_viewport *viewport = &ev->surface->buffer_viewport;
 	struct drm_output *output = (struct drm_output *) output_base;
 
 	if (c->gbm == NULL)
 		return NULL;
 	if (output->base.transform != WL_OUTPUT_TRANSFORM_NORMAL)
 		return NULL;
+	if (viewport->buffer.scale != output_base->current_scale)
+		return NULL;
 	if (output->cursor_view)
 		return NULL;
 	if (ev->output_mask != (1u << output_base->id))