Move toColor into channels

It operates on rgba channels, not a packed 32 bit int

src/common/Types.d.ts

@@ -110,8 +110,8 @@ export interface ICharset {
 export type CharData = [number, string, number, number];
 
 export interface IColor {
-  css: string;
-  rgba: number; // 32-bit int with rgba in each byte
+  readonly css: string;
+  readonly rgba: number; // 32-bit int with rgba in each byte
 }
 export type IColorRGB = [number, number, number];