Form Flatten

[btecu]

Implements form flattening. Closes #554.

[Hopding]

@btecu Wow... this is awesome! Form flattening is a much wanted feature that will help a lot of users. It's probably the biggest missing form-filling feature. Thanks for working on this! I've left a few pieces of feedback for you.

[Hopding]

Looks great. Thanks again @btecu!

[chislin]

When running form.flatten() on PDF saved from result of executing script from this example, I get this error:

 Error: Failed to find page undefined for element favorite.superhero
    at _loop_1 (/node_modules/pdf-lib/cjs/api/form/PDFForm.js:463:27)
    at PDFForm.flatten (/node_modules/pdf-lib/cjs/api/form/PDFForm.js:492:17)

[Hopding]

Looks like this is because the form flattening implementation relies on an optional widget field:

pdf-lib could set this field when adding widgets to pages. But it does seem unfortunate that form flattening would require the presence of an optional field.

[btecu]

@chislin is this the pdf-lib_form_creation_example.pdf file? It only has one page.
I'd like to know if you have a document with more than one page, where a widget is missing the P reference.

For these other cases, we could check if P is missing and the document only has one page, and default to that one.

[chislin]

@btecu attached one
form_on_3rd.pdf

[btecu]

@chislin can you create an issue?