SSSE3 optimizations for gray -> RGBA (or BGRA)
SSSE3 optimizations for gray -> RGBA (or BGRA)

Swizzle Bench Runtime
Dell Venue 8 0.16x
HP z620      0.47x

PNG Decode Time (for test set of gray encoded PNGs)
Dell Venue 8 0.80x
HP z620      0.96x

BUG=skia:4767
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1657393002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1657393002

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