x11-toolkits/fltk: update this port to the latest version 1.3.10

FLTK 1.3.10 is a maintenance release with minor enhancements and
bug fixes. Particularly, Fl_XFont_On_Demand class was previously
not labeled FL_EXPORT, which made it not exposed in the library,
in turn breaking `x11-toolkits/py-fltk' port and other consumers.

There are no new ABI features and thus it is 100% compatible with
releases 1.3.5-1.3.9. This is intended to be the last release of
the 1.3.x series.

PR: 277433, 277439