[Fix] workaround osx 10.6 lost first keypress problem, fix #14525

this PR may fix http://trac.xbmc.org/ticket/14525
there seems be a bug on osx 10.6 only, it will lost the first keypress in when we work with the system input method.
so the problem maybe caused by the way key events was processed by the libsdl, to workaround it, I add a little patch for the osx part of code of libsdl, to let it send the key events directly to our OSXTextInputResponser, so let's test it to see...