GTK4: Add a shadow surface to GtkDrawingArea widgets, which all the drawing functions draw on. When drawing to a widget add it to the dirty list if it doesn't already exist. dw_flush() will trigger a redraw on all dirty widgets, and then clear the dirty list.

[Desktop Entry]
Name=Dynamic Windows Test
Exec=@prefix@/bin/dwtest 
Terminal=false
MultipleArgs=false
Type=Application
Categories=Application;Development;X-Red-Hat-Base;