Normally, we could read gl_Layer from bits 26:16 of R0.0.  However, the
specification requires that bogus out-of-range 32-bit values written by
previous stages need to appear in the fragment shader as-written.

Instead, we pass in the full 32-bit value from the VUE header as an
extra flat-shaded varying.  We have the SF override the value to 0
when the previous stage didn't actually write a value (it's actually
defined to return 0).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>