st/mesa: use table-driven approach to exposing extensions for formats (v3)

st/mesa: use table-driven approach to exposing extensions for formats (v3)

The check for ctx->API was unnecessary, because OES extensions are not exposed
in desktop GL.

Also require renderbuffer support for ARB_texture_rgb10_a2ui,
as per the spec.

Tested by comparing old and new glxinfo with softpipe and r600g.

v2: fix bugs
v3: rename need_only_one -> need_at_least_one
    rename num_elements -> num_mappings
    add comments
    use const when appropriate

Reviewed-by: Brian Paul <brianp@vmware.com>