Copy dest color child index in GrSkSLFP copy constructor

Change-Id: I16c763a0e2634c9c408c25dbf26caf3c28b2ba79
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/487226
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed