Revert "Remove #ifdefs related to SkMesh and SkSL-dependent code."

This reverts commit f30c46d147a59d786a2f8b7318ff05c2da45bc69.

Reason for revert: G3 IWYU issue

Original change's description:
> Remove #ifdefs related to SkMesh and SkSL-dependent code.
>
> The plan in a future CL is to move SkMesh to include/effects
> to be alongside SkRuntimeEffect.
>
> This also enforces IWYU on include/effects/SkImageFilters.h
> and SkMesh.h which may have knock-on effects for clients
>
> Client CLs:
>  - http://cl/547507191
>  - https://crrev.com/c/4679593
>  - https://github.com/flutter/engine/pull/43680
>
> Change-Id: I0a1394f8087cad19d0d4e0fe1be423ffb6aa4e11
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/721978
> Reviewed-by: Brian Osman <brianosman@google.com>

Change-Id: I6209111c7a3f27e979dde891ceeab0b3c71349f4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/724037
Auto-Submit: Brian Osman <brianosman@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
33 files changed