Add DSLVar::swap method.

This gives DSLVar the ability to be set up after initial construction.
To do this, we create a plain empty Var, then swap it with the actual
Var we want afterwards. This allows DSL to support Vars which are
`uniform half4` in some cases and `const half4` (or entirely unused) in
other cases.

This technique was adapted from similar code in Ethan's parser CL.

Change-Id: Ic54d037a0102fda77b25d4755caf77a291eaa8c6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/400716
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed