gs: Check GL errors in primitive-id-restart test.

Previously, if drawing failed due to a GL error, no primitives would
be generated, so the test would have nothing to check, and it would
erroneously pass.  Avoid that by checking for GL errors at the end of
the test.
glut framework: skip if implementation doesn't support required GL version.

In effect, we are already doing this when Waffle is in use, since
Waffle uses a context creation mechanism that ensures that the context
won't be created if the implementation doesn't support the GL version
needed by the test.  But for GLUT we need to do the check manually,
since GLUT always just creates the highest GL version context it can.
gs: Don't try to use gl_PerVertex when redeclaring fragment shader inputs.

In commit 5b1dff3 (Geometry shaders: Test gl_ClipDistance input (GLSL
1.50)) I accidentally used this syntax to redeclare the
gl_ClipDistance fragment shader input:

    in gl_PerVertex {
      float gl_ClipDistance[8];
    };

This is incorrect--gl_PerVertex is only meaningful in vertex and
geometry shaders.  In fragment shaders gl_ClipDistance isn't in any
interface block.

v2: don't forget the "in" keyword.
Test that in/out qualifiers are required when redeclaring gl_ClipDistance.

Although it is not explicitly stated in the GLSL spec, all examples of
redeclaring built-in in/out variables (such as gl_ClipDistance)
include the "in" or "out" qualifier, so it seems like failure to do so
should cause a compile error.

At present:

- Mesa (as of commit a50c5f8) does not generate an error.

- The NVIDIA proprietary driver for Linux (version 313.18) does not
  generate an error; however, if gl_ClipDistance is redeclared in the
  fragment shader without specifying the "in" keyword, it fails to
  work properly.
Test unsized arrays in interface blocks

These tests verify that unsized arrays appearing inside interface
blocks obey the same rules as unsized arrays appearing outside
interface blocks.  Namely, their size is inferred from the largest
constant integer index used to access them across all compilation
units for the same shader stage, and this inferred size is used to
validate that interface blocks are properly matched between one shader
stage and the next.