Cast outline.tags to char * to fix FreeType 2.13.3 build

[fwcd]

Unfortunately, AGG no longer builds with FreeType 2.13.3 as FT_Outline changed some of its members from char * to unsigned char * (compare the 2.13.2 docs with the current docs):

swiftplot/Sources/AGG/agg_font_freetype.cpp:182:35: error: assigning to 'char *' from 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not
            tags  = outline.tags  + first;
                    ~~~~~~~~~~~~~~^~~~~~~

This PR patches the issue by adding a cast, which should work on both 2.13.2 (or older) and 2.13.3. Once fixed upstream, the AGG sources could be updated.