swscale: more generic check for planar destination formats with alpha

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libswscale/swscale.c

@@ -2672,7 +2672,7 @@ static int swScale(SwsContext *c, const uint8_t* src[],
         }
     }
 
-    if ((dstFormat == PIX_FMT_YUVA420P) && !alpPixBuf)
+    if (isPlanar(dstFormat) && isALPHA(dstFormat) && !alpPixBuf)
         fillPlane(dst[3], dstStride[3], dstW, dstY-lastDstY, lastDstY, 255);
 
 #if HAVE_MMX2