Use strict equalities in make.js, external/* and extensions/*
Merge pull request #5119 from timvandermeij/strict-equalities-other

Use strict equalities in make.js, external/* and extensions/*
Use strict equalities in src/core/image.js
Use strict equalities in src/core/jpg.js
Use strict equalities in src/core/jpx.js
Use strict equalities in src/core/jbig2.js
Merge pull request #5121 from timvandermeij/strict-equalities-images

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