Make colorMin an optional argument

When running the example program in the README in TypeScript, I got type-checking errors because it said `colorMin` was required in `renderAmplitudeProgram`.

This change corrects the type definition to make `colorMin` optional.

src/Programs.ts

@@ -389,7 +389,7 @@ void main() {
 	scale?: number,
 	opacity?: number,
 	colorMax?: number[],
-	colorMin: number[],
+	colorMin?: number[],
 	precision?: GLSLPrecision,
 }) {
 	const { type } = params;