avcodec/flashsv2enc: factorize updating block dimensions

avcodec/flashsv2enc: factorize updating block dimensions

The patch changes the init function to initialize block dimensions to fixed
64x64 instead of the previously used image width/height based value.

This should not cause any actual change in behaviour because block dimensions
are recalculated on every keyframe in optimum_block_width() and
optimum_block_height() functions and in the current code the result is always
64x64 regardless of the image dimensions used.

Signed-off-by: Marton Balint <cus@passwd.hu>