gs: Adapt ignore-adjacent-vertices for GLSL 1.50.

To avoid code duplication, I've left the source code for the test in
spec/arb_geometry_shader4, but I've added a command line argument that
causes it to use core GLSL 1.50 functionality rather than
ARB_geometry_shader4.

The new test cases appear in piglit output under spec/glsl-1.50.
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.