Evas GL: Fix sync, wlbuffer and image egl ext functions

Evas GL: Fix sync, wlbuffer and image egl ext functions

Before this patch, those EGL/EvasGL functions can not work
without a current context. But EGL does not require any
current context for those to work, or at least, this should
be left to the driver to decide.

Evas GL was only able to get a pointer to the display
if a context was current.

The display pointer should be infered from Evas_GL unless
we can find a current display. EGL does not require a
context to be current in most of these function calls.

This should bring evasgl a little bit closer to EGL in terms
of behaviour (those are EGL-only extensions, btw).

Thanks @spacegrapher for the quick review

@fix