compositor-drm: Don't use overlay when surface alpha != 1.0f

Assume that overlays can't handle alpha blending even with a constant
alpha factor.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

src/compositor-drm.c

@@ -572,6 +572,9 @@ drm_output_prepare_overlay_surface(struct weston_output *output_base,
 	if (es->buffer == NULL)
 		return NULL;
 
+	if (es->alpha != 1.0f)
+		return NULL;
+
 	if (wl_buffer_is_shm(es->buffer))
 		return NULL;