Support 'layout(when)' expressions in DSL C++.

This required some changes to how we name variables in DSL.
When-expressions are designed to expect a local C++ variable with the
same name as the layout key. This constraint means our DSLVar variables
CANNOT have the same name as the layout key. Now, all DSL variables are
given a prefix. We try to keep the code tidy by using just a leading
underscore as the prefix, where it's safe to do so. (The C++ naming
rules put some underscore-names out of bounds, but underscore followed
by a lowercase letter is safe.)

Change-Id: Iaa8878042329b9909096f05712d5cf636ea01822
Bug: skia:11854
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/400623
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
14 files changed