Clear text layer to the background color on creation

Prevents a black flash before the first draw

src/renderer/BaseRenderLayer.ts

@@ -33,6 +33,10 @@ export abstract class BaseRenderLayer implements IRenderLayer {
     this._canvas.style.zIndex = zIndex.toString();
     this._ctx = this._canvas.getContext('2d', {alpha});
     this._ctx.scale(window.devicePixelRatio, window.devicePixelRatio);
+    // Draw the background if this is an opaque layer
+    if (!alpha) {
+      this.clearAll();
+    }
     container.appendChild(this._canvas);
   }