Enums that were previously handled as bitfields can no longer be extended

[BwackNinja]

This commit changed how Flags are handled, which is cleaner but too restrictive: ac888ae

Unfortunately, that breaks using org.freedesktop.gstreamer.gst.Buffer.map when using the flag GstGL.MAP_GL = 131072, defined in GstGL-1.0.gir. Without that, you cannot read GL frames from gstreamer when using java-gi, which is needed when using a shared context between gtk and gstreamer to display hardware GL video frames.

[jwharm]

Apparently MapFlags used by Buffer.map() is an "open enum" and GstGL adds their own flag constant.

I'll change this specific enum into int constants so that it will work.