Revert "Remove double support from SkSL."

This reverts commit 71a35d49b73448e646c82538ecb009fc68471580.

Reason for revert: ASAN failures on swarming bots

Original change's description:
> Remove double support from SkSL.
> 
> Doubles are not supported by Metal or GLSL pre-4.0, are not supported in
> most backends, and aren't used in any GMs. There isn't any good way to
> use them in new code as it would just degrade to float on many of our
> supported platforms. (This is assuming that our backends actually know
> how to degrade doubles to floats, which is not universally the case.)
> 
> Change-Id: Ieacc69db4bdacca104a15a6eef33e05f977d1ffa
> Bug: skia:10299
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292846
> Commit-Queue: John Stiles <johnstiles@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>

TBR=bsalomon@google.com,ethannicholas@google.com,johnstiles@google.com

Change-Id: I175e42420bcae8dfacd0bfeb269dd84e0b3c9d25
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:10299
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293268
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
10 files changed