Simplify optional input handling in SkMixerColorFilter
Simplify optional input handling in SkMixerColorFilter

When the first input is null, swap with the second - this allows the
implementation to assume CF0 != null.

TBR=
Change-Id: I9b3d0aeecaffed3b56f8956138305b17d33eb1de
Reviewed-on: https://skia-review.googlesource.com/c/193164
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>

aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,google/skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia