Separate HSVA color into HSV, HSVA, fix alpha premultiplication

internal/color.go

@@ -1,13 +1,31 @@
 package internal
 
+import "image/color"
+
+// HSV represents a color with hue, saturation, and value components. Each
+// component C is in range 0 <= C <= 1.
+type HSV struct {
+	H float64
+	S float64
+	V float64
+}
+
+// HSVA is an HSV color with an added 8-bit alpha component. The alpha component
+// ranges from 0x0000 (fully transparent) to 0xFFFF (opaque), and has no bearing
+// on the other components.
 type HSVA struct {
 	H float64
 	S float64
 	V float64
-	A uint8
+	A uint16
 }
 
-func (hsva HSVA) RGBA () (r, g, b, a uint32) {
+var (
+	HSVModel  color.Model = color.ModelFunc(hsvModel)
+	HSVAModel color.Model = color.ModelFunc(hsvaModel)
+)
+
+func (hsv HSV) RGBA () (r, g, b, a uint32) {
 	// Adapted from:
 	// https://www.cs.rit.edu/~ncs/color/t_convert.html