SkSL-to-SkVM: Prepare to handle function calling

Change-Id: I4a130b0c776888587122a7ae25e1b32b10011d55
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/351499
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed