gpu: Use the right format for atlases

The glyph atlas format needs to match the cairo memory format.
NGL was getting this wrong, causing color glyphs to have their
red and blue swapped.

gsk/gpu/gskgldevice.c

@@ -119,7 +119,7 @@ gsk_gl_device_create_atlas_image (GskGpuDevice *device,
   GskGLDevice *self = GSK_GL_DEVICE (device);
 
   return gsk_gl_image_new (self,
-                           GDK_MEMORY_R8G8B8A8_PREMULTIPLIED,
+                           GDK_MEMORY_DEFAULT,
                            GSK_GPU_IMAGE_RENDERABLE,
                            width,
                            height);