SSE 4.1 SrcOver blits: color32, blitmask.
SSE 4.1 SrcOver blits: color32, blitmask.

This is mainly warmup for an AVX2 version.
The machine I'm typing this on just doesn't support AVX2.
This strategy should translate easily down to SSSE3 and SSE2.

Xfermode_SrcOver:    2.73ms -> 2.62ms  (0.96x)   (That's Color32.)
Xfermode_SrcOver_aa: 3.48ms -> 3.09ms  (0.89x)   (That's BlitMask_D32_A8.)

AA text blits (text_16_AA_{88,FF,WT,BK}) show speedups in the range of 5 to 20%.

Unlike previous versions of this code, all the div255() are exactly (x+127)/255.
This won't fix any major bugs, but it does correct our bias in the middle.
There will be many diffs, all minor.

I've punted for now on pmaddubsw for lerping. I do intend to try that,
but I want this (relatively simple) code as my basis for comparison.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1526883004
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1526883004

rubenvb/skia,tmpvar/skia.cc,tmpvar/skia.cc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,tmpvar/skia.cc,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,qrealka/skia-hc,tmpvar/skia.cc,aosp-mirror/platform_external_skia,google/skia,tmpvar/skia.cc,tmpvar/skia.cc,qrealka/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,qrealka/skia-hc,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,tmpvar/skia.cc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,qrealka/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,google/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,qrealka/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,qrealka/skia-hc,aosp-mirror/platform_external_skia,tmpvar/skia.cc,google/skia,HalCanary/skia-hc,rubenvb/skia,google/skia,qrealka/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,qrealka/skia-hc