libvisio: update to 0.1.6.

libvisio 0.1.6
- Consider stencil txtxform in binary formats (tdf#67914).
- Parse font width scaling.
- Fix dependency of tests on time zone (tdf#92396).
- Enable building of unit tests without command-line tools.
- Remove namespace check for VDX to allow parsing documents produced by
  lucidchart.com (tdf#98791).
- Output shape ID if set.
- Fix parsing of text block background color in some cases.
- Write bitmaps with color palette correctly.
- Deduce text encoding from font name.
- Parse theme fill and shadow color.
- Parse font scheme in VSDX documents.
- Require C++11 for build.
- Fix various crashes, leaks and hangs when reading damaged files found
  by oss-fuzz.
- Drop outdated Windows project files.
- Fix some issues found by Coverity.
- Many other small improvements and fixes.

(wiz)