[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/texture.h

@@ -0,0 +1,79 @@
+/* $Id: texture.h,v 1.8 1997/11/13 02:17:32 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: texture.h,v $
+ * Revision 1.8  1997/11/13 02:17:32  brianp
+ * added a few const keywords
+ *
+ * Revision 1.7  1997/05/03 00:53:56  brianp
+ * all-new texture sampling via gl_texture_object's SampleFunc pointer
+ *
+ * Revision 1.6  1997/04/14 02:02:39  brianp
+ * moved many functions into new texstate.c file
+ *
+ * Revision 1.5  1997/02/09 18:53:14  brianp
+ * added GL_EXT_texture3D support
+ *
+ * Revision 1.4  1997/01/09 19:48:58  brianp
+ * added gl_texturing_enabled()
+ *
+ * Revision 1.3  1996/11/14 01:03:09  brianp
+ * removed const's from gl_texgen() function to avoid VMS compiler warning
+ *
+ * Revision 1.2  1996/11/08 02:20:09  brianp
+ * gl_do_texgen() replaced with gl_texgen()
+ *
+ * Revision 1.1  1996/09/13 01:38:16  brianp
+ * Initial revision
+ *
+ */
+
+
+#ifndef TEXTURE_H
+#define TEXTURE_H
+
+
+#include "types.h"
+
+
+extern void gl_texgen( GLcontext *ctx, GLint n,
+                       GLfloat obj[][4], GLfloat eye[][4],
+                       GLfloat normal[][3], GLfloat texcoord[][4] );
+
+
+
+extern void gl_set_texture_sampler( struct gl_texture_object *t );
+
+
+
+extern void gl_texture_pixels( GLcontext *ctx, GLuint n,
+                               const GLfloat s[], const GLfloat t[],
+                               const GLfloat r[],
+                               const GLfloat lambda[],
+                               GLubyte red[], GLubyte green[],
+                               GLubyte blue[], GLubyte alpha[] );
+
+
+#endif
+