display/ui: add a callback to indicate GL state is flushed

Displaying rendered resources requires blocking qemu GPU to avoid extra
framebuffer copies. For an external display, via Spice currently, there
is a callback to block/unblock the rendering in the same thread.

But with the vhost-user-gpu backend, the qemu process doesn't handle
the rendering itself, and the blocking callback isn't effective.
Instead, the backend must be notified when the display code is done.

Fix this by adding a new GraphicHwOps callback to indicate the GL state
is flushed, and we are done manipulating the shared GL resources. Call
it from gtk and spice display.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210204105232.834642-19-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>