glsl: Fix linker checks for GLSL ES 3.00.

glsl: Fix linker checks for GLSL ES 3.00.

This patch updates the following linker checks to do the right thing
in GLSL 3.00 ES:

- Failing to write to gl_Position is allowed in GLSL 1.40+ as well as
  GLSL 3.00 ES.

- It is an error to write to both gl_ClipVertex and gl_ClipDistance in
  GLSL 1.30+.  This does not apply to GLSL 3.00 ES.

- GLSL 3.00 ES uses the same varying counting rules as GLSL 1.00 ES.

- In GLSL 1.30 and GLSL 3.00 ES, "discard" terminates the shader.

- In GLSL 1.00 ES and GLSL 3.00 ES, both a fragment and a vertex
  shader must be present.

[v2, idr]: Fix minro typo in a comment.  Noticed by Ken.

[v3, idr]: s/IsEs(Shader|Prog)/IsES/  Suggested by Ken and Eric.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Carl Worth <cworth@cworth.org>