Add the close() method to the ImageBitmap interface

[junov]

This method releases the graphics resources held by an
ImageBitmap object.
This is part of the larger OffscreenCanvas feature proposal:
https://wiki.whatwg.org/wiki/OffscreenCanvas
The only known implementation at this time is in Firefox:
https://developer.mozilla.org/en-US/docs/Web/API/ImageBitmap/close

[junov]

Drive-by editorial tweak.

[domenic]

The current form is actually preferable, at least for now and in HTML. this technically only has meaning, currently, in ES-style algorithms, so we've been trying to use "this X object" or "this element" instead.

Eventually we may have Web IDL define the this keyword to mean "the Web IDL object corresponding to the JavaScript object given by this" or something like that.

[domenic]

I assume Chrome is interested :). If so, we have two vendors, so LGTM except without the drive-by editorial tweak.

[junov]

Done.

[annevk]

(I was thinking that unsetting the image data does not need to be done since it's not observable, but apparently we already do that elsewhere. Perhaps something we can clean up at some point.)

It is true that the effect of unsetting is not observable in JS, but it is observable for the user. One of the purposes of this API is to force an immediate release of resources instead of waiting for the ImageBitmap to be garbage collected. So being explicit about makes some sense IMHO. From a practical standpoint there are several justifications for this:

1. The ImageBitmap object may be holding on to GPU resources (depending on the implementation) and GPU resource strain is typically not a garbage collection trigger for the JavaScript heap. Solving this problem in the browser implementation is extremely difficult because GPU APIs offer poor visibility into resource availability.
2. Bitmap resource churn in RAM can cause frequent garbage collections, which may result in animation hiccups. This API gives app devs more explicit control to mitigate that issue.
3. Proactive releasing keeps the process size under control, which is good for system-wide performance.