drm-buffer: Keep track of handle as well

This handle is used by the legacy KMS API; lets avoid having to have GBM
specific code where this is done by letting the MetaDrmBuffer API, that
already has this information, expose it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2275>

src/backends/native/meta-drm-buffer.c

@@ -53,6 +53,7 @@ typedef struct _MetaDrmBufferPrivate
   MetaDrmBufferFlags flags;
 
   uint32_t fb_id;
+  uint32_t handle;
 } MetaDrmBufferPrivate;
 
 G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (MetaDrmBuffer, meta_drm_buffer,
@@ -152,6 +153,8 @@ meta_drm_buffer_do_ensure_fb_id (MetaDrmBuffer        *buffer,
     }
 
   priv->fb_id = fb_id;
+  priv->handle = fb_args->handle;
+
   return TRUE;
 }
 
@@ -185,6 +188,14 @@ meta_drm_buffer_get_fb_id (MetaDrmBuffer *buffer)
   return priv->fb_id;
 }
 
+uint32_t
+meta_drm_buffer_get_handle (MetaDrmBuffer *buffer)
+{
+  MetaDrmBufferPrivate *priv = meta_drm_buffer_get_instance_private (buffer);
+
+  return priv->handle;
+}
+
 int
 meta_drm_buffer_get_width (MetaDrmBuffer *buffer)
 {