GL 3.2: Test validity of GL_DEPTH_CLAMP state

Reviewed-by: Paul Berry <stereotype441@gmail.com>
gs: Test that geometry shader inputs must be arrays.

v2: Additional tests.
gs: Test that geometry shader input declaration rules don't apply to outputs.
gs: Test EndPrimitive() functionality.
gs: Make texelFetch and textureSize tests use GLSL 1.50 for geometry shaders.

Previously, these tests used ARB_geometry_shader4.  However, the
initial implementation of geometry shaders in Mesa isn't going to
support ARB_geometry_shader4.  So test this functionality using GLSL
1.50 geometry shaders instead.

v2: Use a generic vertex attribute instead of gl_PositionIn.
!UPSTREAM: Test core GS inputs on GLSL 1.50

TODO:
- Make this work.
util: Add piglit_build_simple_program_unlinked().
Make use of piglit_build_simple_program_unlinked() in some tests.