compositor-drm: skip overlay for shm buffers

They can't be imported by gbm, so no point in trying, and segfaulting
in gbm when we try to import (because it tries to deref the buffer as
a wl_drm_buffer).

Signed-off-by: Rob Clark <rob@ti.com>

src/compositor-drm.c

@@ -599,6 +599,9 @@ drm_output_prepare_overlay_surface(struct weston_output *output_base,
 	if (es->buffer == NULL)
 		return NULL;
 
+	if (wl_buffer_is_shm(es->buffer))
+		return NULL;
+
 	if (!drm_surface_transform_supported(es))
 		return NULL;