mesa: Rename gl_context::swtnl_im to vbo_context; use proper type.

mesa: Rename gl_context::swtnl_im to vbo_context; use proper type.

The main GL context's swtnl_im field is the VBO module's vbo_context
structure.  Using the name "swtnl" in the name is confusing since
some drivers use hardware texturing and lighting, but still rely on the
VBO module for drawing.

v2: Forward declare the type and use that instead of void *
    (suggested by Eric Anholt).
v3: Remove unnecessary cast (pointed out by by Topi Pohjolainen).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>