gpu: Do a GC run after every tile of large images

When we draw large images, we absolutely do not want to keep memory that
we do not need. So do a GC run after every tile. That otentially slows
down things, but it also improves the chances of not running out of
memory.

Here's the node for the image I managed to create after I applied this
patch:

repeat {
  bounds: 0 0 50000 50000;
  child: text {
    font: "Noto Color Emoji 10000px";
    glyphs: 661 0 0 0 color;
    offset: 0 10000;
    hint-style: none;
  }
}

gsk/gpu/gskgpurenderer.c

@@ -326,6 +326,9 @@ gsk_gpu_renderer_fallback_render_texture (GskGpuRenderer        *self,
           gdk_texture_downloader_finish (&downloader);
           g_object_unref (texture);
           g_clear_object (&image);
+
+          /* Let's GC like a madman, we draw oversized stuff and don't want to OOM */
+          gsk_gpu_device_maybe_gc (priv->device);
         }
     }