mesa: Remove many extension enable flags

mesa: Remove many extension enable flags

The following extensions are always enabled, and drivers do not have
to option to disable them:

    GL_ARB_multisample
    GL_ARB_texture_compression
    GL_ARB_vertex_buffer_object / GL_OES_mapbuffer
    GL_EXT_copy_texture
    GL_EXT_multi_draw_arrays / GL_SUN_multi_draw_arrays
    GL_EXT_polygon_offset
    GL_EXT_subtexture
    GL_EXT_texture_edge_clamp / GL_SGIS_texture_edge_clamp
    GL_EXT_vertex_array
    GL_SGIS_generate_mipmap

This set was picked because the are all either required or optional
features in desktop OpenGL, OpenGL ES 1.x, and OpenGL ES 2.x.  The
existing support for some is already partially broken in Mesa (e.g.,
proxy texture targets in OpenGL ES).  This patch does not change the
situation in any way.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>