The case was under macOS with a non-GL parent window mapped to a retina display
containing a GL subwindow and if the app did not call Fl::use_high_res_GL(1).
The new argument gives the window to be captured, or NULL to indicate capture
from the current offscreen.
Calling this function becomes easier because less dependent on global variables.
The procedure to set screen scaling factors becomes:
1) each screen scaling factor is set to 1
2) the OS is queried according to each platform to get screen scaling factor
values
3) The value of FLTK_SCALING_FACTOR, if present, is used to multiply
scaling factors
