x11/fltk13: Update to 1.3.7

This release does not contain new "ABI features". The ABI (Application
Binary Interface) is 100% compatible with releases 1.3.5 and 1.3.6.

CHANGES IN FLTK 1.3.7                            RELEASED: Jul 25 2021

FLTK 1.3.7 is a maintenance release for macOS only. It fixes one
regression introduced in FLTK 1.3.6 and two long standing timer
issues on macOS.

Other platforms than macOS are not concerned.

Changes:

  Avoid premature FL_RELEASE event at start of drag-n-drop operation
  Fix a timer inconsistency and prevent a crash
  Fl::add_timeout() must always create a new timer (#248)

(micha)