fix checkbox support

[FlaverSaver]

[sergiodinizoswald]

@codeshell any chance we can merge this? Just did some tests, not big tests, but in a PDF which I can't write to the checkboxes normally, with this pull request it works perfectly. Thanks!

[brainz80]

same here.

[brainz80]

Noticed that the checkboxes aren't being rendered correctly.

What I'm looking for:

What I'm getting:

The checkboxes are missing their borders and the check-mark isn't centered, it's smaller and looks as if it's a different font.

Maybe this is something @FlaverSaver could take a look at?

[s-kruschel]

This fix works fine in my case, too.

[chasedson]

This works! @codeshell please merge if possible.

Only bug I've noticed - it does not fill successfully if the box has an 'alt' attribute (called 'tooltip' in Acrobat)