Add support for MMType1 fonts with embedded font files

According to the PDF specification (see http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#page=265):
> If the font program for a multiple master font instance is embedded in the PDF file, it shall be an ordinary Type 1 font program, not a multiple master font program. This font program is called a snapshot of the multiple master font instance that incorporates the chosen values of the design coordinates.

Based on the above, I don't think that we actually need to do anything to be able to support PDF files with `MMType1` fonts with embedded font files. In all the cases of `MMType1` fonts that I've come across, all the font properties used are really just the ones used for normal `Type1` fonts.

Fixes #1381.
Fixes #4500.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=794222.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=849634.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1069315.