etnaviv: implement ARB_shadow lowering for halti < 2
This enables the GL_ARB_shadow extension for all GPU models, which unlocks desktop OpenGL version 2.1 for GC2000.
There are some problems with GC3000 regarding ARB_shadow. It should support tex compare but piglit is not happy yet. I am looking into this issue during the next days. I will handle GC3000 in a separate MR.
Supersedes !12915 (closed)
Edited by Christian Gmeiner