fix recording of webgl2 canvases (#106)

webgl2 canvases created after highlight would start recording with the
`preserveDrawingBuffer: false`
setting would not record correctly because we would snapshot a
transparent image of the canvas.
instead of trying to clear the canvas, we should create a context with
`preserveDrawingBuffer: true` to
ensure that the canvas can be recorded (tested in babylon.js)
https://app.highlight.io/1/sessions/n7P0x5XTItCqEN7B7pmtJVldUzJo

packages/rrweb/src/record/index.ts

@@ -331,6 +331,8 @@ function record<T = eventWithTime>(
     blockSelector,
     mirror,
     sampling: sampling?.canvas?.fps,
+    clearWebGLBuffer: sampling?.canvas?.clearWebGLBuffer,
+    initialSnapshotDelay: sampling?.canvas?.initialSnapshotDelay,
     dataURLOptions,
     resizeFactor: sampling?.canvas?.resizeFactor,
     maxSnapshotDimension: sampling?.canvas?.maxSnapshotDimension,