add kRGBA_F16_SkColorType

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1666343002

Review URL: https://codereview.chromium.org/1666343002
diff --git a/include/core/SkColor.h b/include/core/SkColor.h
index 90453f5..b1571c7 100644
--- a/include/core/SkColor.h
+++ b/include/core/SkColor.h
@@ -180,6 +180,11 @@
 
     static SkPM4f FromPMColor(SkPMColor);
 
+    // half-float routines
+    void toF16(uint16_t[4]) const;
+    uint64_t toF16() const; // 4 float16 values packed into uint64_t
+    static SkPM4f FromF16(const uint16_t[4]);
+
 #ifdef SK_DEBUG
     void assertIsUnit() const;
 #else