Windows don't open on Mac if size has not been set

[tirix]

If an application does not call explicitly the window_size method of WindowDesc, the window does not appear on Mac.
This is the case for some of the examples (anim, custom_widget). Others which actually call that method (like calc) don't have the issue.

[tirix]

It seems the issue is with line 166 of druid-shell's window.rs for mac.
The PR #1532 changed that line to set a default windows size to (0,0). If I manually set it back to (500, 400), the issue disappears. However, when I run a dropdown example (still on mac), the drop-down sub-windows briefly appears with a bigger size (most probably 500x400) when opened.
I believe that artefact is less an issue than the fact that some examples seemingly don't work at all. I'd suggest reverting that line and opening a new issue to track the very brief artefact. Ultimately the issue will probably be with finding out the size of the sub-window at the time they are opened.