[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/image.h

@@ -0,0 +1,80 @@
+/* $Id: image.h,v 1.5 1997/09/27 00:15:39 brianp Exp $ */
+
+/*
+ * Mesa 3-D graphics library
+ * Version:  2.5
+ * 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: image.h,v $
+ * Revision 1.5  1997/09/27 00:15:39  brianp
+ * changed parameters to gl_unpack_image()
+ *
+ * Revision 1.4  1997/02/09 20:05:03  brianp
+ * new arguments for gl_pixel_addr_in_image()
+ *
+ * Revision 1.3  1997/02/09 18:52:53  brianp
+ * added GL_EXT_texture3D support
+ *
+ * Revision 1.2  1996/11/06 04:22:23  brianp
+ * changed gl_unpack_image() components argument to srcFormat
+ *
+ * Revision 1.1  1996/09/13 01:38:16  brianp
+ * Initial revision
+ *
+ */
+
+
+#ifndef IMAGE_H
+#define IMAGE_H
+
+
+#include "types.h"
+
+
+extern void gl_flip_bytes( GLubyte *p, GLuint n );
+
+
+extern void gl_swap2( GLushort *p, GLuint n );
+
+extern void gl_swap4( GLuint *p, GLuint n );
+
+
+extern GLint gl_sizeof_type( GLenum type );
+
+
+extern GLint gl_components_in_format( GLenum format );
+
+
+extern GLvoid *gl_pixel_addr_in_image( struct gl_pixelstore_attrib *packing,
+                                const GLvoid *image, GLsizei width,
+                                GLsizei height, GLenum format, GLenum type,
+                                GLint row);
+
+
+extern struct gl_image *gl_unpack_image( GLcontext *ctx,
+                                  GLint width, GLint height,
+                                  GLenum srcFormat, GLenum srcType,
+                                  const GLvoid *pixels );
+
+
+extern void gl_free_image( struct gl_image *image );
+
+
+#endif