winsys-glx: map X window automatically if not foreign

winsys-glx: map X window automatically if not foreign

For now we are going for the semantics that when a CoglOnscreen is first
allocated then it will automatically be mapped. This is for convenience
and if you don't want that behaviour then it is possible to instead
create an Onscreen from a foreign X window and in that case it wont be
mapped automatically.

This approach means that Cogl doesn't need onscreen_map/unmap functions
but it's possible we'll decide later that we can't avoid adding such
functions and we'll have to change these semantics.