Measure the single advance width with a heuristic method

This fix is needed to use dual-width fonts, which have double-width
glyphs (e.g. CJK unified ideographs).

Signed-off-by: Ryusei Yamaguchi <mandel59@gmail.com>
Signed-off-by: Christoph Lohmann <20h@r-36.net>

config.def.h

@@ -417,3 +417,11 @@ static uint selmasks[] = {
 	[SEL_RECTANGULAR] = Mod1Mask,
 };
 
+/*
+ * Printable characters in ASCII, used to estimate the advance width
+ * of single wide characters.
+ */
+static char ascii_printable[] =
+	" !\"#$%&'()*+,-./0123456789:;<=>?"
+	"@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_"
+	"`abcdefghijklmnopqrstuvwxyz{|}~";