glsl: Add a lowering pass for gl_VertexID

glsl: Add a lowering pass for gl_VertexID

Converts gl_VertexID to (gl_VertexIDMESA + gl_BaseVertex). gl_VertexIDMESA
is backed by SYSTEM_VALUE_VERTEX_ID_ZERO_BASE, and gl_BaseVertex is backed
by SYSTEM_VALUE_BASE_VERTEX.

v2: Put the enum in struct gl_constants and propoerly resolve the scope
in C++ code.  Fix suggested by Marek.

v3: Reabase on Matt's foreach_in_list changes (was using foreach_list).

v4 (Ken): Use a systemvalue instead of a uniform because
STATE_BASE_VERTEX has been removed.

v5: Use a boolean to select lowering, and only allow one lowering
method.  Suggested by Ken.

v6 (Ken): Replace strcmp against literal "gl_BaseVertex"/"gl_VertexID"
with SYSTEM_VALUE enum checks, for efficiency.

v7: Rebase on context constant initialization work.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>