gallium: Add context profile support to st_api.

gallium: Add context profile support to st_api.

Add struct st_context_attribs to describe context profiles and
attributes.  Modify st_api::create_context to take the new struct
instead of an st_visual.

st_context_attribs can be used to support GLX_ARB_create_context_profile
and GLX_EXT_create_context_es2_profile in the future.  But the
motivation for doing it now is to be able to replace ST_API_OPENGL_ES1
and ST_API_OPENGL_ES2 by profiles.

Having 3 st_api's to provide OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is
not a sane abstraction, since all of them share glapi for current
context/dispatch management.