[graphite] Rename dst color arg field to fRuntimeShaderDstColor

In Graphite I think we'd like for "dstColor" to consistently mean a
color from the framebuffer. Since this arg field represents a different
concept it would be nice to have this extra qualifier.

Change-Id: I69945ff08e879d0329a6e058c6cbad49c6045e25
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/651976
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed