replace custom/libimagequant Color and Palette types with the identical go stdlib ones

Color.go

@@ -1,5 +0,0 @@
-package imagequant
-
-type Color struct {
-	r, g, b, a uint8
-}

Palette.go

@@ -1,29 +0,0 @@
-package imagequant
-
-/*
-#include "libimagequant.h"
-*/
-import "C"
-
-// This struct has standard Go lifetime and does not need manual release.
-type Palette struct {
-	p C.struct_liq_palette
-}
-
-func (this *Palette) Count() uint {
-	return uint(this.p.count)
-}
-
-func (this *Palette) At(idx int) (Color, error) {
-	if idx < 0 || idx >= int(this.Count()) {
-		return Color{}, ErrValueOutOfRange
-	}
-
-	return Color{
-		r: uint8(this.p.entries[idx].r),
-		g: uint8(this.p.entries[idx].g),
-		b: uint8(this.p.entries[idx].b),
-		a: uint8(this.p.entries[idx].a),
-	}, nil
-
-}

Result.go

@@ -1,5 +1,9 @@
 package imagequant
 
+import (
+	"image/color"
+)
+
 /*
 #include "libimagequant.h"
 */
@@ -68,9 +72,21 @@ func (this *Result) WriteRemappedImage() ([]byte, error) {
 	return buff, nil
 }
 
-func (this *Result) GetPalette() *Palette {
-	ptr := *C.liq_get_palette(this.p) // copy struct content
-	return &Palette{p: ptr}
+func (this *Result) GetPalette() color.Palette {
+	ptr := C.liq_get_palette(this.p) // copy struct content
+	max := int(ptr.count)
+
+	ret := make([]color.Color, max)
+	for i := 0; i < max; i += 1 {
+		ret[i] = color.RGBA{
+			R: uint8(ptr.entries[i].r),
+			G: uint8(ptr.entries[i].g),
+			B: uint8(ptr.entries[i].b),
+			A: uint8(ptr.entries[i].a),
+		}
+	}
+
+	return ret
 }
 
 // Free memory. Callers must not use this object after Release has been called.