Fix const globals in Metal

We were emitting this at global scope (not in Globals). That would lead
to errors about the variable needing to be in the constant address
space. (You can see the result in ConstArray.metal - the old code was
invalid). Also, we were already making references use _globals, so the
code was double-wrong (or half-right, depending on your perspective).

After the core change, writeVarDeclaration was only used for local
scope, and writeModifiers never used the 'globalContext' parameter.

The removal of finishLine() changed every test output, unfortunately.

Change-Id: Icc1356ba2cc3c339b2f5759b3d18523fd39395bc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/408356
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
173 files changed