Tiny pgtk regression

[John Yates]

My current OS is Ubuntu 21.10.  I build emacs from master
and run with a black background via Prot's modus-vivendi.

Historically I specify --with-x-toolkit=athena.  Recently I have
been experimenting with --with-x-toolkit=pgtk.  Previously, at
start the initial frame appeared already filled with the black
background.  Now, after changing only the toolkit, the initial
frame flashes white and then changes to black.