Check isEvalSupported, and test that eval is actually supported, before attempting to use the PostScriptCompiler (issue 5573)

Currently `PDFFunction` is implemented (basically) like a class with only `static` methods. Since it's used directly in a number of different `src/core/` files, attempting to pass in `isEvalSupported` would result in code that's *very* messy, not to mention difficult to maintain (since *every* single `PDFFunction` method call would need to include a `isEvalSupported` argument).

Rather than having to wait for a possible re-factoring of `PDFFunction` that would avoid the above problems by design, it probably makes sense to at least set `isEvalSupported` globally for `PDFFunction`.

Please note that there's one caveat with this solution: If `PDFJS.getDocument` is used to open multiple files simultaneously, with *different* `PDFJS.isEvalSupported` values set before each call, then the last one will always win.
However, that seems like enough of an edge-case that we shouldn't have to worry about it. Besides, since we'll also test that `eval` is actually supported, it should be fine.

Fixes 5573.

src/core/evaluator.js

@@ -54,6 +54,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
     disableFontFace: false,
     nativeImageDecoderSupport: NativeImageDecoding.DECODE,
     ignoreErrors: false,
+    isEvalSupported: true,
   };
 
   function NativeImageDecoder(xref, resources, handler, forceDataSchema) {