Add SkSL module `sksl_shared`.

This module lives directly atop the root module; `sksl_gpu` and
`sksl_public` descend from it.

This module allows us to deduplicate common code, such as the common
SkSL intrinsic list. Sharing the intrinsic list will reduce our memory
consumption, and having a place for Ganesh and Graphite common code to
live will reduce needless duplication going forward.

Change-Id: I6fc8d1c8d77d22b619b629c7d38279001a3f20c9
Bug: skia:13540
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/562304
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
15 files changed