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).
Hi, 
Section B. 6 specifies how to divide a resolution level into rectangular blocks called precincts, and section B.7 specifies how to divide a subband into rectangular blocks called codeblocks. Surprisingly, there is no direct specification of how to map codeblocks into precincts. That is why the Jasper library had to establish a new term, called ***Codeblock-Group*** to derive the correct mapping, and I have decided to use this variable name. 
For this reason I have to disagree with you about the naming, but I will change it if you should insist. 

Please note that `subbandCodeblockWidth/Height` is a misleading variable name, since it does not describe neither the codeblock dimensions nor the subbands' dimensions.
What it should describe is the number of samples in a subband that map to a whole precinct.
Maybe a more descriptive name is `subbandSamplesPerPrecinctWidth/Height` ?

Paul