popup: a border, a preedit as wide as its text, one place by the pointer, and no dying

The popup was a white box on the usually white text it covered, with
no edge to tell them apart; it has a border in the separator's colour.
A syllable being composed came in a bar twelve ems wide, wiping out the
line under it: only candidate rows share that steady width now, a
preedit alone hugs its text.  A popup placed by the pointer, when the
client sends no caret, was placed again on every key and so followed
the mouse; it stays where it came up.  Clicks on it no longer fall
through to the root window and its menu.  And one failed draw — a lost
pointer reply, a pixmap the server refused — ended the draw thread and
the popup for the rest of the session; only a dead connection does now.

tests/popup_test.c

@@ -144,10 +144,10 @@ popup_layout(struct ct *t)
 	CT_EQ_INT(t, -1, p.rowsy);
 	CT_EQ_INT(t, -1, p.marky);
 
-	/* Preedit only. */
+	/* Preedit only: as wide as its text. */
 	dc.pre = mkstr("preedit");
 	popuplayout(&dc, 2*PopupBasew, Imgh, &p);
-	CT_EQ_INT(t, PopupBasew, p.w);
+	CT_EQ_INT(t, textwidth(&dc.pre) + 2*PopupPad, p.w);
 	CT_EQ_INT(t, PopupPad, p.prey);
 	CT_EQ_INT(t, -1, p.sepy);
 	CT_EQ_INT(t, -1, p.rowsy);