Add factories for finer color management

src/drawables/empty-drawable.ts

@@ -0,0 +1,18 @@
+import { mat2d, vec2 } from 'gl-matrix';
+import { Drawable } from './drawable';
+import { DrawableDescriptor } from './drawable-descriptor';
+
+/**
+ * @internal
+ */
+export class EmptyDrawable extends Drawable {
+  public static readonly descriptor: DrawableDescriptor;
+
+  public minDistance(target: vec2): number {
+    throw new Error('Unimplemented');
+  }
+
+  protected getObjectToSerialize(transform2d: mat2d, transform1d: number): any {
+    throw new Error('Unimplemented');
+  }
+}