swscale/swscale_internal: fix ya16 -> rgba

Fixes Ticket644

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale_internal.h

@@ -778,6 +778,8 @@ static av_always_inline int isALPHA(enum AVPixelFormat pix_fmt)
         || (x)==AV_PIX_FMT_YVYU422     \
         || (x)==AV_PIX_FMT_UYVY422     \
         || (x)==AV_PIX_FMT_YA8       \
+        || (x)==AV_PIX_FMT_YA16LE      \
+        || (x)==AV_PIX_FMT_YA16BE      \
         ||  isRGBinInt(x)           \
         ||  isBGRinInt(x)           \
     )