[Nel] Linux compilation pb

[Vincent Caron]

I couldn't compile src/3d/driver/opengl/driver_opengl_texture.cpp, it was
lacking the prototype for a GL extension function. I found out that "gl.h"
includes "glext.h". And "glx.h" include "gl.h". And so on ... The conclusion
is that you must define GL_GLEXT_PROTOTYPES (if needed) before *any* GL
inclusion. I took this block from driver_opengl_extension.h :

#ifdef NL_OS_UNIX
#define	GL_GLEXT_PROTOTYPES
#endif

and moved it to the very beginning of driver_opengl.h (before the #include
<GL/glx.h>).