anv: Fix VK_EXT_transform_feedback working with gl_Layer
Transform feedback did not handle that VARYING_SLOT_LAYER and VARYING_SLOT_VIEWPORT are packed into VARYING_SLOT_PSIZ.
I'm not sure that
VARYING_SLOT_VIEWPORT should be handled here
since Vulkan spec for VK_EXT_transform_feedback states:
RESOLVED: Transform feedback cannot be made active in a render pass with multiview enabled.
- How should queries be done?
However could this varying exist in shader without multiview being enabled?
The issue with VARYING_SLOT_LAYER was found in RenderDoc since it uses transform feedback to fetch GS outputs.