Draft: etnaviv: implement ARB_shadow lowering for halti <= 2
The BASELOD COMPARE_ENABLE and COMPARE_FUNC fields don't exist on GC2000. Unless I'm missing something, they don't function on GC3000 either.
We can emulate shadow texture loads with compare function for halti <= 2 with shader variants.
This fixes the piglit test spec@arb_depth_texture@texdepth on GC3000. On GC2000, this enables the GL_ARB_shadow extension, which unlocks desktop OpenGL version 2.1.