Overwrite invalid rendermodes with default

Supposedly[1] the fallback path is called often enough
to cause performance degradation with invalid values;
fix up those values so this path is only taken once.

[1]: libass#118 (comment)

Author: TheOneric

src/post-worker.js

@@ -70,6 +70,7 @@ self.getRenderMethod = function () {
             return self.render;
         default:
             console.error('Unrecognised renderMode, falling back to default!');
+            self.renderMode = 'wasm-blend';
             // fallthrough
         case 'wasm-blend':
             return self.blendRender;