[skottie] 'Add' layer blend mode support
[skottie] 'Add' layer blend mode support

BM now exports an enum for 'add' blending [1]. This corresponds to
SkBlendmode::kPlus.


[1] https://github.com/bodymovin/bodymovin-extension/commit/86874c10180106e362f6db0997033a8f85fff999

Change-Id: I3c9a52e4055563f50bb68f460eda27fed1c4a411
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208220
Auto-Submit: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>

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