glapi: Rename ARB_compute_shader "LOCAL" -> "WORK_GROUP".

The piglit glapi files, which are derived from files published on
www.opengl.org, contained nonstandard names for two enums:
COMPUTE_LOCAL_WORK_SIZE and MAX_COMPUTE_LOCAL_INVOCATIONS.

The ARB_compute_shader spec, the OpenGL specs, and the OpenGL wiki all
agree that these should be called COMPUTE_WORK_GROUP_SIZE and
MAX_COMPUTE_WORK_GROUP_INVOCATIONS.  So rename them in piglit to be
consistent.
util: Add ARB_compute_shader support to piglit_get_gl_enum_name().
WIP: minmax test for ARB_compute_shader.