Softpipe add support for TGSI_OPCODE_INTERP_*

This MR adds support for the TGSI_OPCODE_INTERP_* opcodes that implement the according interpolateAt* functions with the limitation that softpipe doesn't truly support MSAA, i.e. it advertises only support for one sample.

The series also raises the GLSL level to 400 which exposes more features that are already implemented. With that piglit run gpu -x glx enables 1370 tests of with 9 fail and 4 crash (The crashes are all related to transform feedback with GS shaders, the according tests with VS shaders also crash).

I've put the series on top of !702 (merged), because there are many texture fixes that I think should also land before raising of the GLSL level.