[OPENGL32][MESA] Downgrade Mesa library to version 2.6

With this commit, we now use a forked version of MESA which only supports OpenGL 1.1, like the windows implementation does.
It exposes :
  - The same pixel formats
  - The same set of extensions
  - Nothing more
All of this without taking 10% of your build time.
If you need a more modern option, look at the MESA package from Rapps, which is (and must be) maintained outside of this code tree.
CORE-7499

dll/opengl/mesa/api.h

@@ -0,0 +1,83 @@
+/* $Id: api.h,v 1.4 1998/02/04 00:38:24 brianp Exp $ */
+
+/*
+ * Mesa 3-D graphics library
+ * Version:  2.4
+ * Copyright (C) 1995-1997  Brian Paul
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the Free
+ * Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+
+/*
+ * $Log: api.h,v $
+ * Revision 1.4  1998/02/04 00:38:24  brianp
+ * WIN32 patch from Oleg Letsinsky
+ *
+ * Revision 1.3  1998/02/04 00:13:35  brianp
+ * updated for Cygnus (Stephane Rehel)
+ *
+ * Revision 1.2  1997/11/25 03:20:09  brianp
+ * simple clean-ups for multi-threading (John Stone)
+ *
+ * Revision 1.1  1997/08/22 01:42:26  brianp
+ * Initial revision
+ *
+ */
+
+
+/*
+ * The original api.c file has been split into two files:  api1.c and api2.c
+ * because some compilers complained that api.c was too big.
+ *
+ * This header contains stuff only included by api1.c and api2.c
+ */
+
+
+#ifndef API_H
+#define API_H
+
+
+/*
+ * Single/multiple thread context selection.
+ */
+#ifdef THREADS
+
+/* Get the context associated with the calling thread */
+#define GET_CONTEXT	GLcontext *CC = gl_get_thread_context()
+
+#else
+
+/* CC is a global pointer for all threads in the address space */
+#define GET_CONTEXT
+
+#endif /* THREADS */
+
+
+/*
+ * An optimization in a few performance-critical functions.
+ */
+#define SHORTCUT
+
+
+/*
+ * Windows 95/NT DLL stuff.
+ */
+#if !defined(WIN32) && !defined(WINDOWS_NT) && !defined(__CYGWIN32__)
+#define APIENTRY
+#endif
+
+
+#endif