avcodec/codec_internal: Use union for FFCodec decode/encode callbacks

This is possible, because every given FFCodec has to implement
exactly one of these. Doing so decreases sizeof(FFCodec) and
therefore decreases the size of the binary.
Notice that in case of position-independent code the decrease
is in .data.rel.ro, so that this translates to decreased
memory consumption.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/8bps.c

@@ -180,7 +180,7 @@ const FFCodec ff_eightbps_decoder = {
     .p.id           = AV_CODEC_ID_8BPS,
     .priv_data_size = sizeof(EightBpsContext),
     .init           = decode_init,
-    .decode         = decode_frame,
+    FF_CODEC_DECODE_CB(decode_frame),
     .p.capabilities = AV_CODEC_CAP_DR1,
     .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };