Test that all desktop GLSL versions can be cross-linked.

Verified using the NVIDIA proprietary driver for Linux (version
313.18).
gs: Test that gl_MaxGeometryInputComponents can actually be achieved.
gs: Fix geometry layout case insensitivity tests.

These tests use check_link: true, which means that they each need to
specify a complete set of geometry shader layout qualifiers in order
to pass.
gs: Test redeclaring either gl_PerVertex in or out, and using both.
Test a corner case involving arrays, structs, and interface blocks.

This patch exercises the same corner case as
https://bugs.freedesktop.org/show_bug.cgi?id=70368.