PGTK default color for unrendered frame

Hello,

When Emacs (compiled with --with-pgtk) gets resized on a Wayland 
compositor (in this case, Labwc), it shows a white artifact for a split 
second, caused by PGTK filling the unrendered part of the frame with a 
default color (usually white).
I was thinking whether we could expose an Elisp PGTK function that lets 
Emacs change this color at runtime, in order for it to better fit the 
chosen color theme.

- Adam