Builds a ElementFrameCapturer that rasterizes an element to a base64 JPEG (no data-URL prefix).
The returned function never throws. It returns null (so the pump sends no frame) when the element has no size, is too
large, a capture is already in flight, a capture times out or fails, or capture has been disabled after repeated or
permanent failure; in the last two cases the reason is reported once through OnDegraded.
Builds a ElementFrameCapturer that rasterizes an element to a base64 JPEG (no data-URL prefix).
The returned function never throws. It returns
null(so the pump sends no frame) when the element has no size, is too large, a capture is already in flight, a capture times out or fails, or capture has been disabled after repeated or permanent failure; in the last two cases the reason is reported once throughOnDegraded.