Refactored DSL variable creation

Prior to this change, we were creating the SkSL variable and declaration
immediately on DSLVar creation. This causes problems with function
parameters, which are sometimes supposed to be tagged
SK_MAIN_COORDS_BUILTIN. If we have already created the variable and
inserted it into the symbol table, then by the time we determine the
variable is supposed to be SK_MAIN_COORDS_BUILTIN, it is too late to
modify the (now-const) variable.

We are not yet doing this tagging, but refactoring the creation in this
fashion paves the way to making it possible.

Change-Id: I031170502c5e7c1fff5ecfac01bea470ff4e61ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/389216
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
10 files changed