Get back on mainline libpng 1.6 now that 3bpp perf patches are merged.
Get back on mainline libpng 1.6 now that 3bpp perf patches are merged.

Checked manually in Instruments that we're still running our SSE filters for both RGB and RGBA PNGs.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1865633002

Review URL: https://codereview.chromium.org/1865633002

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