Broke DSLWriter into two separate classes

As DSLWriter evolved, it ended up performing two unrelated duties:

1. Holding per-thread state which is referenced throughout SkSL
   (including non-DSL code)
2. DSL-specific utility functions

There's no reason for those to be contained in the same class, and
there's also no reason the per-thread state should be held in a
DSL-specific class. This breaks out an SkSL::ThreadContext class to hold
the state info, leaving the DSL utility functions behind in DSLWriter
(which should perhaps be renamed, but that's a job for a future CL).

Change-Id: Iccd45314bd9b37d4a1d3e27920e32a50c0b07d7c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/456736
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
26 files changed