Allow setting a trace coordinate without a skvm::Builder.

SkRuntimeEffect users will be insulated from skvm and the Builder, so
they won't be able to make an skvm::Coord directly. Replace this with
an SkIPoint, and do the conversion inside the SkVM code generator.

This also removes the requirement of assigning a trace coordinate; if
no trace coordinate is set, (0, 0) will be used.

Change-Id: I97be025d0ea146a86655cc3cdb76726f3bbc2324
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474556
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed