[skottie] Fix analytical mask fill type

Sksg::Merge needs to preserve the fill type of the first path appended
in the stack.

Theoretically, one could append multiple paths with different fill types
using sksg::Merge, but in practice Skottie should never do that (append
mode with invertible shape only used for the very first mask in a stack).

TBR=
Change-Id: Ie9ac9187cc1c8baaae2bef439313a7700407f04a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303582
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
2 files changed