gl: Make gdk_gl_context_make_current() return void

gl: Make gdk_gl_context_make_current() return void

Its not really reasonable to handle failures to make_current, it
basically only happens if you pass invalid arguments to it, and
thats not something we trap on similar things on the X drawing side.

If GL is not supported that should be handled by the context creation
failing, and anything going wrong after that is essentially a critical
(or an async X error).