Use strict equalities in src/core/{image, jpx, jpg, jbig2}.js

/cc @Snuffleupagus for review.

Some useful test files are:
- http://people.reed.edu/~ahm/Courses/Reed-POL-372-2011-S3_IEP/Syllabus/EReadings/07.1/07.1.EhrlichHoldren1971-03-26Impact.pdf (for `jbig2.js`)
- http://dspace.mit.edu/bitstream/handle/1721.1/48833/planningsystemsc00rama.pdf (for `jpx.js`)
- http://vs.twonky.dk/invitation.pdf (for `jpg.js`)