bug#16001: [5ef95e85] fails to build with --enable-gcc-warnings in some configurations

[martin rudalics]

> There are lot of new errors in window.c, xterm.c and xdisp.c after the 
> pixelwise resize checkin, so this report is kind of obsolete.

Juanma already fixed some of them, I tried to fix the rest.  Please have a look.

Thanks, martin