draw/clip: fix viewport index for geometry shaders

draw/clip: fix viewport index for geometry shaders

The old code updated the viewport index on the first vertex in
a primitive, however it was picking the first vertex wrong
when used with geometry shaders.

This code has access to the prim info with the primitive lengths
so instead keep track of when a new primitive starts by tracking
the lengths and updating the viewport index then. The prim info
is only valid after a GS or prim assembly, so enable prim assembly
if a vertex shader ever uses viewport index.

This fixes:
piglit arb_viewport_array-render-viewport-2
KHR-GLES31.core.viewport_array.draw_to_single_layer_with_multiple_viewports,Fail
KHR-GLES31.core.viewport_array.draw_mulitple_viewports_with_single_invocation,Fail
KHR-GLES31.core.viewport_array.draw_multiple_layers,Fail
KHR-GLES31.core.viewport_array.depth_range,Fail

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5489>