llvmpipe: enable PIPE_CAP_TGSI_VS_LAYER_VIEWPORT
llvmpipe: enable PIPE_CAP_TGSI_VS_LAYER_VIEWPORT

No changes required in the driver itself, all handled by draw.

piglit results in a quick run:
skip->pass 7
skip->fail 2
(The new failures in the ARB_fragment_layer_viewport group are expected,
we fail the same if gs doesn't write these outputs regardless of the vs.)

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler