The cache has been added in #3312 in 2013 and a lot of things changed since.
Having too many cached accelerated canvases can lead to have to move their data from the GPU to the RAM
which is costly.
So this patch:
- removes all the cached canvases;
- destroys the useless canvases in order to free their associated memory asap;
- slightly rewrite canvas.js::_scaleImage to avoid too much canvas creation.
- Over time the number and size of these factories have increased, especially the `DOMFilterFactory` class, and this split should thus aid readability/maintainability of the code.
- By introducing a couple of new import maps we can avoid bundling the `DOMCMapReaderFactory`/`DOMStandardFontDataFactory` classes in the Firefox PDF Viewer, since they are dead code there given that worker-thread fetching is always being used.
- This patch has been successfully tested, by running `$ ./mach test toolkit/components/pdfjs/`, in a local Firefox artifact-build.
*Note:* This patch reduces the size of the `gulp mozcentral` output by `1.3` kilo-bytes, which isn't a lot but still cannot hurt.
