Rather than constructing a transformation matrix piece by piece (with
very branchy code), draw_stretch_before now just calls cairo_translate()
and cairo_scale() with values that are almost-obviously correct.
Also, rather than stashing and restoring the transformation matrix
ourselves, it seems simpler to use cairo_save() and cairo_restore().
That requires that draw_stretch_before() and draw_stretch_after() be
called strictly in pairs, but they are so that's OK.
Correcting a source file name in the docs just now reminded me that
I've seen a lot of outdated source file names elsewhere in the code,
due to all the reorganisation since we moved to cmake. Here's a giant
pass of trying to make them all accurate again.
This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.
