font/sfnt: use charmap EncodeRune API.
font/sfnt: use charmap EncodeRune API.

Change-Id: I91d850d1816b43748580f6f8a8f19151472750d0
Reviewed-on: https://go-review.googlesource.com/39070
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Run-TryBot: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/image