EGL: Implement the libglvnd interface for EGL (v3)

The new interface mostly just sits on top of the existing library.

The only change to the existing EGL code is to split the client
extension string into platform extensions and everything else. On
non-glvnd builds, eglQueryString will just concatenate the two strings.

The EGL dispatch stubs are all generated. The script is based on the one
used to generate entrypoints in libglvnd itself.

v2: [Kyle]
 - Rebased against master.
 - Reworked the EGL makefile to use separate libraries
 - Made the EGL code generation scripts work with Python 2 and 3.
 - Change gen_egl_dispatch.py to use argparse for the command line arguments.
 - Assorted formatting and style cleanup in the Python scripts.

v3: [Emil Velikov]
 - Rebase
 - Remove separate glvnd glx/egl configure toggles

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

configure.ac

@@ -1348,10 +1348,11 @@ dnl Libglvnd configuration
 dnl
 AC_ARG_ENABLE([libglvnd],
     [AS_HELP_STRING([--enable-libglvnd],
-        [Build for libglvnd @<:@default=disabled@:>@])],
+        [Build GLX and EGL for libglvnd @<:@default=disabled@:>@])],
     [enable_libglvnd="$enableval"],
     [enable_libglvnd=no])
-AM_CONDITIONAL(USE_LIBGLVND_GLX, test "x$enable_libglvnd" = xyes)
+AM_CONDITIONAL(USE_LIBGLVND, test "x$enable_libglvnd" = xyes)
+
 if test "x$enable_libglvnd" = xyes ; then
     dnl XXX: update once we can handle more than libGL/glx.
     dnl Namely: we should error out if neither of the glvnd enabled libraries
@@ -1367,8 +1368,11 @@ if test "x$enable_libglvnd" = xyes ; then
         ;;
     esac
 
-    PKG_CHECK_MODULES([GLVND], libglvnd >= 0.1.0)
-    DEFINES="${DEFINES} -DUSE_LIBGLVND_GLX=1"
+    PKG_CHECK_MODULES([GLVND], libglvnd >= 0.2.0)
+    PKG_CHECK_VAR(LIBGLVND_DATADIR, libglvnd, datadir)
+    AC_SUBST([LIBGLVND_DATADIR])
+
+    DEFINES="${DEFINES} -DUSE_LIBGLVND=1"
     DEFAULT_GL_LIB_NAME=GLX_mesa
 fi