Do not change codec in flv streams if the user has forced a codec.

Fixes ticket #2218.
vp56: remove clear_blocks call, and clear alpha plane U/V DC only.

The non-alpha and alpha-Y planes are cleared in the idct_put/add()
calls. For the alpha U/V planes, we only care about the DC for entropy
context prediction purposes, the rest of the data is unused.

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