Fix DynamicCharAtlas with transparency

There were two bugs:
- We need to clear the background of _tmpCtx instead of just drawing on
  top if the background color is transparent.
- We should set the background to fully transparent if it's partially
  transparent, to avoid drawing the transparent background twice.

src/renderer/Renderer.ts

@@ -150,6 +150,7 @@ export class Renderer extends EventEmitter implements IRenderer {
   }
 
   public onOptionsChanged(): void {
+    this.colorManager.allowTransparency = this._terminal.options.allowTransparency;
     this._runOperation(l => l.onOptionsChanged(this._terminal));
   }