Implement FromImageColor

Converts from `image/color.Color` to `tcell.Color`.

color.go

@@ -14,7 +14,10 @@
 
 package tcell
 
-import "strconv"
+import (
+	ic "image/color"
+	"strconv"
+)
 
 // Color represents a color.  The low numeric values are the same as used
 // by ECMA-48, and beyond that XTerm.  A 24-bit RGB value may be used by
@@ -1066,4 +1069,13 @@ func GetColor(name string) Color {
 // PaletteColor creates a color based on the palette index.
 func PaletteColor(index int) Color {
 	return Color(index) | ColorValid
-}
+}
+
+// FromImageColor converts an image/color.Color into tcell.Color.
+// The alpha value is dropped, so it should be tracked separately if it is
+// needed.
+func FromImageColor(imageColor ic.Color) Color {
+	r, g, b, _ := imageColor.RGBA()
+	// NOTE image/color.Color RGB values range is [0, 0xFFFF] as uint32
+	return NewRGBColor(int32(r>>8), int32(g>>8), int32(b>>8))
+}

color_test.go

@@ -15,6 +15,7 @@
 package tcell
 
 import (
+	ic "image/color"
 	"testing"
 )
 
@@ -110,3 +111,19 @@ func TestColorRGB(t *testing.T) {
 		t.Errorf("RGB wrong (%x, %x, %x)", r, g, b)
 	}
 }
+
+func TestFromImageColor(t *testing.T) {
+	red := ic.RGBA{0xFF, 0x00, 0x00, 0x00}
+	white := ic.Gray{0xFF}
+	cyan := ic.CMYK{0xFF, 0x00, 0x00, 0x00}
+
+	if hex := FromImageColor(red).Hex(); hex != 0xFF0000 {
+		t.Errorf("%v is not 0xFF0000", hex)
+	}
+	if hex := FromImageColor(white).Hex(); hex != 0xFFFFFF {
+		t.Errorf("%v is not 0xFFFFFF", hex)
+	}
+	if hex := FromImageColor(cyan).Hex(); hex != 0x00FFFF {
+		t.Errorf("%v is not 0x00FFFF", hex)
+	}
+}