glsl: look for frag data bindings with [0] tacked onto the end for arrays

glsl: look for frag data bindings with [0] tacked onto the end for arrays

The GL spec is very unclear on this point. Apparently this is discussed
without resolution in the closed Khronos bugtracker at
https://cvs.khronos.org/bugzilla/show_bug.cgi?id=7829 . The
recommendation is to allow dropping the [0] for looking up the bindings.

The approach taken in this patch is to instead tack on [0]'s for each
arrayness level of the output's type, and doing the lookup again. That
way, for

out vec4 foo[2][2][2]

we will end up looking for bindings for foo, foo[0], foo[0][0], and
foo[0][0][0], in that order of preference.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96765
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Eric Anholt <eric@anholt.net>