Avoid opaque canvases in case of Firefox (fixes #117)

src/sample.ts

@@ -15,6 +15,7 @@ import {
 	toDataView,
 	toUint8Array,
 	SetRequired,
+	isFirefox,
 } from './misc';
 
 /**
@@ -228,7 +229,10 @@ export class VideoSample {
 			}
 
 			const canvas = new OffscreenCanvas(width, height);
-			const context = canvas.getContext('2d', { alpha: false, willReadFrequently: true });
+			const context = canvas.getContext('2d', {
+				alpha: isFirefox(), // Firefox has VideoFrame glitches with opaque canvases
+				willReadFrequently: true,
+			});
 			assert(context);
 
 			// Draw it to a canvas
@@ -343,7 +347,7 @@ export class VideoSample {
 			dest.set(this._data);
 		} else {
 			const canvas = this._data;
-			const context = canvas.getContext('2d', { alpha: false });
+			const context = canvas.getContext('2d');
 			assert(context);
 
 			const imageData = context.getImageData(0, 0, this.codedWidth, this.codedHeight);