winsys-glx: Remove the vblank counter feature when indirect rendering
winsys-glx: Remove the vblank counter feature when indirect rendering

Previously when Cogl detects that the GLX context is indirect it
resets the function pointers for the VBLANK_COUNTER feature to NULL.
However it wasn't removing the VBLANK_COUNTER feature flag. Some other
parts of the winsys check for that feature flag rather than checking
whether the pointer is NULL so it would end up calling an invalid
function pointer and crashing. This just fixes it to also clear the
feature flag.

https://bugzilla.gnome.org/show_bug.cgi?id=684917

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

gcampax/cogl,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl