Good form type detection is important to get reliable telemetry and to
only show the fallback bar if a form cannot be filled out by the user.
PDF.js only supports AcroForm data, so XFA data is explicitly unsupported
(tracked in issue #2373). However, the previous form type detection
couldn't separate AcroForm and XFA well enough, causing form type
telemetry to be incorrect sometimes and the fallback bar to be shown for
forms that could in fact be filled out by the user.
The solution in this commit is found by studying the specification and
the form documents that are available to us. In a nutshell the rules are:
- There is XFA data if the `XFA` entry is a non-empty array or stream.
- There is AcroForm data if the `Fields` entry is a non-empty array and
it doesn't consist of only document signatures.
The document signatures part was not handled in the old code, causing a
document with only XFA data to also be marked as having AcroForm data.
Moreover, the old code didn't check all the data types.
Now that AcroForm and XFA can be distinguished, the viewer is configured
to only show the fallback bar for documents that only have XFA data. If
a document also has AcroForm data, the viewer can use that to render the
form. We have not found documents where the XFA data was necessary in
that case.
Finally, we include unit tests to ensure that all cases are covered and
move the form type detection out of the `parse` function so that it's
only executed if the document information is actually requested
(potentially making initial parsing a tiny bit faster).
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