Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

It allows to give them a category (vout display).

The code is based on CheckAPI from egl.c

It is more compatible than to use glXGetProcAddress() which is not always
present (need opengl >= 1.4).

It avoids tearing.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Otherwise Qt4 selects another value by default and fails.

It should fix at least the win32 segfaults.

Yes, some options are not yet recognized; this will change in future commits.
Yes, it ain't perfect; don't cry now.
It is a bit different from what's been done before to actually work with
sh.

They still aren't perfectly compliant though...

DBus, XOSD and Audio Scrobbler interfaces depend on this.

Also protect concurrent access to p_sys between main thread and libvlc callbacks

fixes: #4827