GdkX11GLContext: Use bitblit for swap if no buffer age support

GdkX11GLContext: Use bitblit for swap if no buffer age support

If buffer age is undefined and the updated area is not the whole
window then we use bit-blits instead of swap-buffers to end the
frame.

This allows us to not repaint the entire window unnecessarily if
buffer_age is not supported, like e.g. with DRI2.