Templatize SkPMFloat to support both 1 and 255 biases.

src/core/SkPMFloat.h

Index: src/core/SkPMFloat.h
diff --git a/src/core/SkPMFloat.h b/src/core/SkPMFloat.h
index 4a5621f5b3cc62cf6ae11b5173d8f449fcce10ef..46288323071b5032e05c805cfba45aafb07e2145 100644
--- a/src/core/SkPMFloat.h
+++ b/src/core/SkPMFloat.h
@@ -21,8 +21,11 @@
 namespace {
 
 // A pre-multiplied color storing each component in the same order as SkPMColor,
-// but as a float in the range [0, 1].
+// but as a float in the range [0, kBias],
+// where kBias is either 1 (easier to work with) or 255 (sometimes a little faster).
+template <int kBias>
 class SkPMFloat : public Sk4f {
+    static_assert(kBias == 1 || kBias == 255, "");
 public:
     static SkPMFloat FromPMColor(SkPMColor c) { return SkPMFloat(c); }
     static SkPMFloat FromARGB(float a, float r, float g, float b) { return SkPMFloat(a,r,g,b); }
@@ -47,10 +50,10 @@ public:
     float g() const { return this->kth<SK_G32_SHIFT / 8>(); }
     float b() const { return this->kth<SK_B32_SHIFT / 8>(); }
 
-    SkPMColor round() const;  // Rounds from [0.0f, 1.0f] to [0, 255], clamping if out of range.
+    SkPMColor round() const;  // Rounds from [0, kBias] to [0, 255], clamping if out of range.
 
     bool isValid() const {
-        return this->a() >= 0 && this->a() <= 1
+        return this->a() >= 0 && this->a() <= kBias
             && this->r() >= 0 && this->r() <= this->a()
             && this->g() >= 0 && this->g() <= this->a()
             && this->b() >= 0 && this->b() <= this->a();