turnip: multiViewport and VK_EXT_shader_viewport_index_layer
I had to guess some things because the blob doesn't support either
VK_EXT_shader_viewport_index_layer, but presumably the HW supports it for DX. There might still be some details to work out, but the relevant dEQP tests are happy.
@cwabbott0 note there was a mistake using
VARYING_SLOT_LAYER instead of
VARYING_BIT_LAYER for the
layer_zero key in turnip