popup: one text drawing call; drop what the draw thread never reaches

font.c exported three wrappers for one function, and the fit == -1 mode
existed only for the tests; textdraw() takes fit and colour. win.c
tested the empty picture in the thread and again in winshow, interned
two atoms by hand next to getatom(), kept a consumer-side drain that the
producer's already guarantees never finds anything, zeroed ten globals
before returning from the only thread that read them, and re-checked
sizes that resizebacking had just established. The page marker is laid
out once and drawn from the layout instead of being recomputed.

dat.h

@@ -158,6 +158,7 @@ struct Popup
 	int	markx;
 	int	marky;
 	int	markw;
+	Str	mark;
 	int	selx;
 	int	sely;
 	int	selw;