[vtkusers] Changes to VTK configuration (Mesa and OSMesa related)

[Berk Geveci]

Hello,

I've been cleaning up some of the CMake cache entries related
to Mesa and OSMesa. Here are the entries I am thinking 
adding/changing:

Mesa + OSMesa:
Switch on/off with:
VTK_OPENGL_HAS_OSMESA 
Requires:
OSMESA_INCLUDE_DIR
OSMESA_LIBRARY

OpenGL + Mangled Mesa:
Switch on/off with:
VTK_USE_MANGLED_MESA
Requires:
MANGLED_MESA_INCLUDE_DIR (now points to where GL/gl_mangle.h is)
MANGLED_MESA_LIBRARY
MANGLED_OSMESA_LIBRARY

Essentially, the names of some entries will change and users
of Mesa or mangled Mesa will have to adjust their configurations
accordingly. Let me know if you have any suggestions or
objections.

-Berk