gallivm/llvmpipe: added lp_rast_shader_inputs::facing and pass through

gallivm/llvmpipe: added lp_rast_shader_inputs::facing and pass through

The triangle rasterizer sets this field to indicate front/back-facing.
It gets passed into the generated fragment code as another parameter.
Used now for stencil front/back selection but will also be used for
fragment shaders in general (see TGSI_SEMANTIC_FACE).

With this commit two-sided stenciling mostly works but there's
still a bug or two...