avfilter/vf_overlay: add YUVA422P to alpha_pix_fmts

Now yuv422 output format gives similar expected output as other output formats.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_overlay.c

@@ -301,7 +301,7 @@ fail:
 }
 
 static const enum AVPixelFormat alpha_pix_fmts[] = {
-    AV_PIX_FMT_YUVA420P, AV_PIX_FMT_YUVA444P,
+    AV_PIX_FMT_YUVA420P, AV_PIX_FMT_YUVA422P, AV_PIX_FMT_YUVA444P,
     AV_PIX_FMT_ARGB, AV_PIX_FMT_ABGR, AV_PIX_FMT_RGBA,
     AV_PIX_FMT_BGRA, AV_PIX_FMT_NONE
 };