Use fewer multiplications for Ycck to Rgb conversion

There are two commits in this PR:

* The first commit removes unnecessary null checks: The function [readBit()](https://github.com/mozilla/pdf.js/blob/master/src/core/jpg.js#L113) does either return an integer or throws an exception, but it never returns null. Thus, we do not need to check if `readBit() === null`.

* The second commit does the rgb conversion without the intermediate variables CbCb, CbCr, ... and with less multiplications. Thus, #3809 is a little faster

browser        | Count | Baseline(ms) | Current(ms) |  +/- |    %  | Result(P<.05)
-------------- | ----- | ------------ | ----------- | ---- | ----- | -------------
firefoxNightly |     5 |         8989 |        8110 | -879 | -9.78 |        faster