Fix the ngl downloader

The image is already associated with the original texture, the
one that the download op creates is purely to hand it to the
downloader, it should not be associated with the image.

This makes the ngl downloader work as well as the vulkan one
(ie rgba dmabufs work, yuv ones don't, since we don't convert
them).

gsk/gpu/gskgpudownloadop.c

@@ -358,9 +358,6 @@ gsk_gpu_download_op_gl_command (GskGpuOp          *op,
                                                 gsk_gl_texture_data_free,
                                                 data);
 
-  gsk_gpu_image_toggle_ref_texture (self->image, self->texture);
-  gsk_gl_image_steal_texture_ownership (GSK_GL_IMAGE (self->image));
-
   g_object_unref (builder);
 
   return op->next;