...so that later passing the OStringBuffer's aNum[i] to
createSevenSegmentPolyPolygon (taking a first parameter of type char) doesn't
need to implicitly convert from sal_Unicode to char.

Requires addition of some missing OStringBuffer-related function variants in
rtl/math.hxx and rtl/strbuf.hxx.

Change-Id: I79e6b2a791abc62b6556a6668e4411cced490c11