Cairo and GTK plugins still need fix for narrow window creation.

Both the cairo and GTK libdecor plugins enforce a minimum width for decorated windows. There's still a case, involving the maximum width set for a window, that is not correctly handled by the plugins when the user asks for creation of a narrow window and the plugin sets the effective window width to the minimum admissible value.

Here is, attached, a modified version of libdecor's demo program that creates the not-properly supported situation and shows the fix solves the problem : demo-mod-narrow.c

A separate, similar MR has been opened at jadahl/libdecor!104 for the corresponding change in the cairo plugin.