Fix some signedness issues in the X11 primitives.

Fix some signedness issues in the X11 primitives.

Times are always unsigned; reflect this.

Screen coordinates are signed, and not always nonnegative; project
negative ones onto the border.  Widths and heights are technically
signed too, according to my Xlib.h, but I haven't observed negative
values, and negative values can't possibly make sense, whereas I have
observed negative coordinates in the wild (button down in-window,
button up out-of-window).