xlib: add support for GLX_ARB_create_context

xlib: add support for GLX_ARB_create_context

This adds the glXCreateContextAttribsARB() function for the xlib/swrast
driver.  This allows more piglit tests to run with this driver.

For example, without this patch we get:
$ bin/fbo-generatemipmap-1d -auto
piglit: error: waffle_config_choose failed due to WAFFLE_ERROR_UNSUPPORTED_
ON_PLATFORM: GLX_ARB_create_context is required in order to request an OpenGL
version not equal to the default value 1.0
piglit: error: Failed to create waffle_config for OpenGL 2.0 Compatibility Context
piglit: info: Failed to create any GL context
PIGLIT: {"result": "skip" }

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Acked-by: Roland Scheidegger <sroland@vmware.com>