Add PWG options structure for writing PWGs.

This should (pretty much) give us enough to write a mupdftoraster
equivalent of gstoraster.
Split PWG code into a separate file.
Mudraw: Add -c colorspace flag.
Remove useless -lpng from doc/example.c and doc/multi-threaded.c
Add monochrome PWG output routines.
Addition of support for password input.

Also rework of startup based upon password entry. Fix for bug that
caused a crash when slider was used, thumbs were still rendering and a
new document was opened.
Fix app breakages.

Android app was still thinking that we had only one fz_device_rgb rather
than one per context.

Android app had a typo in resulting in searching ending in an infinite
loop.

Text search failed to search outside the first block of each page.
Android: Fix crash when selecting text.

We were failing to setup and maintain the span number correctly.