Make BaseRenderLayer's _alpha private again

This was accidentally left over from some earlier changes I was playing
with.

src/renderer/BaseRenderLayer.ts

@@ -26,7 +26,7 @@ export abstract class BaseRenderLayer implements IRenderLayer {
     private _container: HTMLElement,
     id: string,
     zIndex: number,
-    protected _alpha: boolean,
+    private _alpha: boolean,
     protected _colors: IColorSet
   ) {
     this._canvas = document.createElement('canvas');