mesa: Implement GL_ARB_polygon_offset_clamp

Semantically identical to the EXT version (whose string is still valid
for GLES), so rename the bit but expose both extension strings.
(Suggested by Ilia Mirkin and Ian Romanick.)

v3: Fix the entrypoint alias in GL4x.xml (Ilia)

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

docs/features.txt

@@ -226,7 +226,7 @@ GL 4.6, GLSL 4.60
   GL_ARB_gl_spirv                                       in progress (Nicolai Hähnle, Ian Romanick)
   GL_ARB_indirect_parameters                            DONE (nvc0, radeonsi)
   GL_ARB_pipeline_statistics_query                      DONE (i965, nvc0, radeonsi, softpipe, swr)
-  GL_ARB_polygon_offset_clamp                           not started
+  GL_ARB_polygon_offset_clamp                           DONE (i965, nv50, nvc0, r600, radeonsi, llvmpipe, swr)
   GL_ARB_shader_atomic_counter_ops                      DONE (i965/gen7+, nvc0, radeonsi, softpipe)
   GL_ARB_shader_draw_parameters                         DONE (i965, nvc0, radeonsi)
   GL_ARB_shader_group_vote                              DONE (i965, nvc0, radeonsi)

docs/relnotes/17.3.0.html

@@ -44,6 +44,7 @@ Note: some of the new features are only available with certain drivers.
 </p>
 
 <ul>
+<li>GL_ARB_polygon_offset_clamp on i965, nv50, nvc0, r600, radeonsi, llvmpipe, swr</li>
 <li>GL_ARB_transform_feedback_overflow_query on radeonsi</li>
 <li>GL_ARB_texture_filter_anisotropic on i965, nv50, nvc0, r600, radeonsi</li>
 <li>GL_EXT_memory_object on radeonsi</li>