android_window: don't clear full subtitle area
android_window: don't clear full subtitle area

Do a memset only on previous area.
Subtitle Display is now 15% - 25% faster (depends on window resolution).

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc