Instead of the stub winsys being a special case set of #ifdef'd code
used when COGL_HAS_FULL_WINSYS wasn't defined, the stub winsys now
implements a CoglWinsysVtable like all other winsys backends (it's just
that everything is a NOP). This way we can get rid of the
COGL_HAS_FULL_WINSYS define and also the stub winsys can be runtime
selected whereas before it was incompatible with all other winsys
backends.
Until Cogl gains native win32/OSX support this remove the osx and win32
winsys files and instead we'll just rely on the stub-winsys.c to handle
these platforms. Since the only thing the platform specific files were
providing anyway was a get_proc_address function; it was trivial to
simply update the clutter backend code to handle this directly for now.
Commit b061f737 moved _cogl_winsys_has_feature to the common winsys
code so there's no need to define it in the stub winsys any more. This
was breaking builds for backends using the stub winsys.
This migrates all the GLX window system code down from the Clutter
backend code into a Cogl winsys. Moving OpenGL window system binding
code down from Clutter into Cogl is the biggest blocker to having Cogl
become a standalone 3D graphics library, so this is an important step in
that direction.
