Test full framebuffer clear color with mismatched layer count.

GL 3.2 allows different framebuffer attachments to have different
layer counts.  Rendering to layers that do not exist in all
attachments is undefined; however, clearing the framebuffer should
still clear all layers of all attachments.
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.