mesa: GL_ARB_shading_language_100 is not optional

mesa: GL_ARB_shading_language_100 is not optional

This extension just provides some of the most basic software framework
for GLSL.  Without GL_ARB_vertex_shader or GL_ARB_fragment_shader,
applications still cannot use GLSL.  There's no value in
conditionalizing support for this extension.

NOTE: This has the side effect of enabling the extension in the radeon,
r200, and nouveau drivers.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>