mesa/st: enable AMD_vertex_shader_viewport_index

The assumption is that any driver capable of emitting layer from the
vertex shader and supporting viewports should be able to also handle
emitting viewport index from the vertex shader.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Tobias Droste <tdroste@gmx.de>
3 files changed