Menus have white corners when background is dark and scheme is gtk+ or plastic on macOS

[ggarra13]

The menus under macOS only show white corners even when the background color is dark under gtk+ or plastic schemes. Find attached a simple demo program to compile with fltk-config --compile.

white_corners.zip

[ggarra13]

I am attaching a patch for gtk+ that fixes the problem on macOS (which has white backgrounds by default, while Linux and WIN32 have black backgrounds). I still have to see plastic and gleam.
gtk_white_corners.txt

[ggarra13]

Here's the fix for plastic, too.
plastic_white_corners.txt

[ggarra13]

And here's the fix for gleam (we fill a rectangle, instead of drawing lines).
gleam_white_corners.txt

[ManoloFLTK]

@ggarra13 : This should now be fixed using the code you proposed at PR #507 with commit 3ba0904. Please, close this issue if you confirm it's fixed. Thanks for your input.

N.B. A typo in the description of commit 3ba0904 makes it refer erroneously to an unrelated issue instead of this issue.