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.

tests/popup_test.c

@@ -74,10 +74,6 @@ popup_layout(struct ct *t)
 		CT_EQ_INT(t, strlen(markers[i].want), n);
 		CT_CHECK(t, strcmp(markers[i].want, buf) == 0);
 	}
-	CT_EQ_INT(t, 12*Fontsz, PopupTextw);
-	CT_EQ_INT(t, 2*PopupPad + PopupNumw + PopupTextw, PopupBasew);
-	CT_EQ_INT(t, 2*PopupPad + (1 + Maxdisp + 1)*Fontsz + PopupSep,
-		Imgh);
 
 	/* Monitor selection retains origins and intersects the EWMH work area. */
 	mon[0] = (Area){0, 0, 1920, 1080};