Declare sk_InvocationID programmatically

This lets us declare it correctly depending on caps, rather than editing
the modifiers of a (shared) global variable.

Change-Id: Ifcdc5e0a8e9b852685ee03a46b537b986b5d35de
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325623
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed