mesa: add support for AMD_vertex_shader_viewport_index

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Tobias Droste <tdroste@gmx.de>
glsl: add support for AMD_vertex_shader_viewport_index

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Tobias Droste <tdroste@gmx.de>
i965: expose AMD_vertex_shader_viewport_index on gen7+

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
r600g: allow vs to write to gl_ViewportIndex

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Tobias Droste <tdroste@gmx.de>
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>
Tested-by: Tobias Droste <tdroste@gmx.de>
radeonsi: add support for vs viewport output

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>