gpu: Make the device queryable from the cache

That's needed in cached subclasses during destruction, when they want to
destroy system resources.

gsk/gpu/gskgpucache.c

@@ -785,6 +785,12 @@ static const GskGpuCachedClass GSK_GPU_CACHED_GLYPH_CLASS =
 /* }}} */
 /* {{{ GskGpuCache */
 
+GskGpuDevice *
+gsk_gpu_cache_get_device (GskGpuCache *self)
+{
+  return self->device;
+}
+
 /*
  * gsk_gpu_cache_set_time:
  * @self: a `GskGpuCache`

gsk/gpu/gskgpucacheprivate.h

@@ -57,6 +57,7 @@ GskGpuCache *           gsk_gpu_cache_new                               (GskGpuD
 gpointer                gsk_gpu_cached_new                              (GskGpuCache            *cache,
                                                                          const GskGpuCachedClass *class);
 
+GskGpuDevice *          gsk_gpu_cache_get_device                        (GskGpuCache            *self);
 void                    gsk_gpu_cache_set_time                          (GskGpuCache            *self,
                                                                          gint64                  timestamp);