Initial work on progressive loading

We are testing this using a new -p flag to mupdf that sets a bitrate at
which data will appear to arrive progressively as time goes on. For
example:

  mupdf -p 102400 pdf_reference17.pdf

Details of the scheme used here are presented in docs/progressive.txt
Import curl submodule.
Add mupdf-curl app

Windows and X11. Allows files to be fetched and displayed
as they are downloaded both with and without linearization, using
hints if available.
Bug 694431: Fix potential infinite loop.

Thanks to "willus" for pointing out this problem.
Collect together all code to do with appearance-stream creation
Update thirdparty/curl to include curl_config.h

This solves linux build problems.
Added newlines in error messages for X11 platform.
Add in missing files into winrt project from 15b9cdd84caf784abeb911ca7aaf1d317d201e88
Fix MIN/MAX confusion in edge list clipping.

The case with infinite scissor didn't work.
Bug 694429: Fix potential overflows in sprintf in pdfextract

Thanks to Pengsu Cheng for pointing out the problem.
Update CHANGES file with recent events.
Fix MSVC Release build.
Fix leak of pdf_field_value.

pdf_js_setup_event strdups the field value, then nothing ever uses it
again.
Fix memory overwrites when plotting glyphs that are completely clipped.

This bug has been in here for ages, but was masked by a bug in the
gel clipping that was fixed by Tor earlier.
Bug 694402: Fix various SVG output problems

Images were missing a space, hence giving errors.

SVG text gets confused by leading (and repeated) whitespace, so elide it.
Fix mutool poster operation.
Add OpenXPS mime-type to fz_open_document.
Clean up whitespace.
Silence 'set but not used' warnings.
Narrow definition of whitespace according to XML spec.

Thanks to Tor for finding the correct definitions for me.