Start precompiling DDL shaders

This is through a private API but it, at least, connects things end-to-end.

Bug: skia:9455
Change-Id: Ib34d49c5c4e4cfa5fa599afc5c967fcadc3de10e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268627
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed