glsl: Generate compile errors for explicit blend indices < 0 or > 1.

glsl: Generate compile errors for explicit blend indices < 0 or > 1.

According to the GLSL 4.30 specification, this is a compile time error.
Earlier specifications don't specify a behavior, but since 0 and 1 are
the only valid indices for dual source blending, it makes sense to
generate the error.

Fixes (the fixed version of) piglit's layout-12.frag.

NOTE: This is a candidate for the 9.0 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>