egl: Support per-thread info.

This commit introduces a "current" system to manage per-thread info.  It
uses TLS, if GLX_USE_TLS is defined, or pthread, if PTHREADS is defined.
If none of them are defined, it uses a dummy implementation that is just
like before.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>

src/egl/main/egldisplay.h

@@ -45,10 +45,6 @@ extern void
 _eglSaveDisplay(_EGLDisplay *dpy);
 
 
-extern _EGLDisplay *
-_eglGetCurrentDisplay(void);
-
-
 extern void
 _eglCleanupDisplay(_EGLDisplay *disp);