WIP: runtime switch for how to interpret SkColor -vs- srgb

Still very conflicted about the "right" way to proceed with this, but thought I'd experiment with a runtime flag, so we can practice seeing SKPs in various stages of "srgb correctness".

Other aspects to either fix, or at least provide runtime switches for:

- untagged images
- gradients
- colorshader
- drawVertices

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1891013002

TBR=

Review URL: https://codereview.chromium.org/1891013002
diff --git a/src/core/SkColor.cpp b/src/core/SkColor.cpp
index ab63300..1c6f0b6 100644
--- a/src/core/SkColor.cpp
+++ b/src/core/SkColor.cpp
@@ -9,6 +9,8 @@
 #include "SkColorPriv.h"
 #include "SkFixed.h"
 
+bool gTreatSkColorAsSRGB;
+
 SkPMColor SkPreMultiplyARGB(U8CPU a, U8CPU r, U8CPU g, U8CPU b) {
     return SkPremultiplyARGBInline(a, r, g, b);
 }
@@ -156,6 +158,11 @@
     Sk4f value = SkNx_shuffle<3,2,1,0>(SkNx_cast<float>(Sk4b::Load(&c)));
     SkColor4f c4;
     (value * Sk4f(1.0f / 255)).store(&c4);
+    if (gTreatSkColorAsSRGB) {
+        c4.fR = srgb_to_linear(c4.fR);
+        c4.fG = srgb_to_linear(c4.fG);
+        c4.fB = srgb_to_linear(c4.fB);
+    }
     return c4;
 }