mesa/st: handle gl_Layer input semantic

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
gallium: add cap to show that fs can accept layer/viewport inputs

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50: enable ARB_fragment_layer_viewport

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