Handling jpx images with multiple precincts per resolution level

As discussed in the comment thread for [issue 5475] (https://github.com/mozilla/pdf.js/issues/5475)
introduced a small change at how codeblocks of each resolution are mapped to their precincts,
for the non-degenerate case. 
Has been tested with large images (5K x 3K approx.) RGB, multiple quality layers, 6 resolution levels
with the following test cases:

1. No precincts at all (single default precinct for each resolution level)
2. Multiple precincts per resolution level with varying sizes for each resolution
3. Precincts with a size larger than the image (should be the similar to the degenerate case).
Can you rename `cblkGroupWidth` to `subbandCodeblockWidth` and  `cblkGroupHeight` to `subbandCodeblockHeight`?

Per "B.7 Division of the sub-bands into code-blocks": "The code-block size for each sub-band at a particular resolution level is determined as 2^xcb ́ by 2^ycb ́ ...."