...assuming these values are indeed intended to be negative (and <https:
//developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6.html#Types>)
mentions a "Fixed: 16.16-bit sgined fixed-point number" type, apparently
represented here by F16Dot16, so the assumption looks plausible).

During CppunitTest_vcl_pdfexport, encountered negative values -1184 and -22 with
instdir/share/fonts/truetype/Carlito-Regular.ttf and -8 with
instdir/share/fonts/truetype/LiberationSans-Bold.ttf.

Change-Id: Ia07c61aebc2c9c67d0ed6173ecac94bab2abb2a8