popup: keep only computed geometry in Popup; one fallback for slivers

numx, textx and selx were compile-time constants stored per layout;
the number column starts at PopupPad and text at PopupPad+PopupNumw,
which dat.h now says once. The height fallback tried dropping the
marker, then the preedit, then the padding as three separate refits;
below one padded row it now shows as many bare rows as fit, at least
one, which is what those steps added up to.

dat.h

@@ -142,6 +142,11 @@ struct Drawcmd
 	Caret	caret;
 };
 
+/*
+ * Popup geometry from popuplayout: n rows from row0 of the candidates,
+ * each Fontsz tall; a section's y is -1 when it is absent.  Numbers
+ * start at PopupPad, text at PopupPad+PopupNumw.
+ */
 typedef struct Popup Popup;
 struct Popup
 {
@@ -152,14 +157,11 @@ struct Popup
 	int	prey;
 	int	sepy;
 	int	rowsy;
-	int	numx;
-	int	textx;
 	int	textw;
-	int	markx;
 	int	marky;
+	int	markx;
 	int	markw;
 	Str	mark;
-	int	selx;
 	int	sely;
 	int	selw;
 };