turnip: shader viewport handling
When trying to run e.g., KHR-GL46.shader_viewport_layer_array.ShaderViewportIndexTestCase
on zink, I hit an assert at ir3_nir_lower_tess.c:111 because VARYING_SLOT_VIEWPORT isn't handled. Given that VK_EXT_shader_viewport_index_layer
is supported, this functionality should work.