Always declare sk_FragColor in GLSL, even if unused

PLS and discard-only shaders are the only time this has an impact,
and it doesn't seem like a problem to have the declaration?

Removes one use of variable reference counts, which are going to be
refactored.

Change-Id: Idb8d06087eed56070252ee02dcf907bf0d24c5a1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328796
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed