popup: follow GDK_SCALE on HiDPI

The popup was 32-pixel rows whatever the display; next to 2x application
text it was small. Its metrics now scale with the daemon's GDK_SCALE,
the setting the applications use.

tests/font_test.c

@@ -5,12 +5,13 @@
 enum
 {
 	Guard = 8,
-	Testw = 6 * Fontsz,
-	Testh = 3 * Fontsz,
-	Testn = Testw * Testh,
 	Rgbmask = 0xffffff,
 };
 
+#define Testw	(6 * Fontsz)
+#define Testh	(3 * Fontsz)
+#define Testn	(Testw * Testh)
+
 static u32int guardcolor = 0x5a5a5a5a;
 
 static void