Replace Metal `constant` keyword with `const`.

"Constant" is an address space qualifier and can't be applied to a
local variable. "Const" in GLSL (and hypothetically SkSL) is meant to
apply to a constant expression regardless of address space.

Our previous test was not finding any error because the optimizer was
eliminating the constant expressions entirely.

Change-Id: I6cfe8e2a621c79945b33e0166780d81e79890a1b
Bug: skia:11304
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368517
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed