cogl/renderer: Set error in dmabuf constructor fallback

When the CoglRenderer didn't set the DMA buffer constructor vfunc, we
return NULL. What we didn't do was set the error, meaning the caller
would crash if it tried to look up why DMA buffer construction failed.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1430

cogl/cogl/cogl-renderer.c

@@ -31,6 +31,7 @@
 
 #include "cogl-config.h"
 
+#include <gio/gio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -768,5 +769,8 @@ cogl_renderer_create_dma_buf (CoglRenderer  *renderer,
   if (winsys->renderer_create_dma_buf)
     return winsys->renderer_create_dma_buf (renderer, width, height, error);
 
+  g_set_error (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
+               "CoglRenderer doesn't support creating DMA buffers");
+
   return NULL;
 }