Handle merging ops tasks with color clears better

This fixes the reduceOpsTaskSplitting bot.

Bug: skia:10877
Change-Id: Ief0343f5ce62121edd8f87aa57c68b74b736bd80
Docs-Preview: https://skia.org/?cl=363941
Cq-Include-Trybots: luci.skia.skia.primary:Test-Debian10-Clang-NUC7i5BNK-GPU-IntelIris640-x86_64-Debug-All-ASAN_ReduceOpsTaskSplitting_Vulkan
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/363941
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
2 files changed