Protects PDFJS.disableXXX == true from change

... when preferences read
Given that the default value is `true`, see https://github.com/mozilla/pdf.js/blob/master/src/display/api.js#L122-L123, shouldn't this have `=== false` instead?
With this PR merged, WebGL is now disabled even this have bee this made setting the pref in about:config