This fixes two things that I overlooked in PR 21225, more specifically:
- Use proper, rather than semi, private class fields in `WasmImage`.
- Make tracking of `WasmImage` instances optional, to avoid keeping data alive permanently in the `IMAGE_DECODERS` build.
Given that these classes are, with the exception of their `decode` methods, virtually identical this helps reduce code duplication and simplifies maintenance.
These changes reduce the size of the `gulp mozcentral` build-target by `1292` bytes, which obviously isn't a lot but still cannot hurt.
*Note:* This is similar to PR 19525, which did the same thing for the OpenJPEG decoder.
The advantages of doing this are:
- The same JBig2 decoder is used regardless of WASM being supported or not, which means consistent rendering.
- The old `Jbig2Image` implementation has various bugs and missing features.
- Less code that needs to be maintained in the PDF.js project, since both the CCITT and the JBig2 decoder is replaced.
The disadvantage of doing this is:
- Slightly larger bundle size, however the effect is limited since a fair amount of PDF.js code can be removed. For the `gulp mozcentral` target the size increase is approximately 54 kilo-bytes (which is small compared to the 452 kilo-bytes for the JS version of the OpenJPEG decoder).
