ByteCode: Improved ExternalValue interface

Use float* to match the ByteCode run API (and make the sizing of data
clearer). Add a lane index to all external value calls. My upcoming
overhaul of the particle code needs this, but I wanted to break that
(large) CL up.

Change-Id: I0588cd7769a1dced9f088de5756947bb744c146b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223178
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed