mesa: Install a minimal dispatch table during glBegin()/glEnd().

mesa: Install a minimal dispatch table during glBegin()/glEnd().

This is a step toward getting rid of ASSERT_OUTSIDE_BEGIN_END() in Mesa.

v2: Finish create_beginend_table() comment, move loopback API init into it,
    and add a const flag. (suggestions by Brian)

Reviewed-by: Brian Paul <brianp@vmware.com> (v1)
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)