Rename globalShareContext to qt_gl_global_share_context

This makes it easier to forward-declare the exported symbol from
other Qt modules without having to include the private headers.

This keeps the old API until dependent submodules are updated.

Change-Id: I08310a684b79f2f612f2ce897a601ff74178bee6
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>