use ExReadFile() for ReadYUV()
use ExReadFile() for ReadYUV()

this allows read-from-stdin.

+ report error on invalid -s option

Change-Id: I0cbb2e7ab95442f9fa5891b47fc3ab71ca07d78b

redengineer/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,redengineer/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,webmproject/libwebp,redengineer/libwebp,redengineer/libwebp,ttyangf/libwebp,PKRoma/libwebp,redengineer/libwebp