ffv1: Explicitly name the coder type

FFv1 uses two types of coders, golomb and range with two different
tables. This is exposed this in a rather convoluted way, for example
mentioning to set coder type 1 while initializing the variable 'ac' to 2,
because encoder does not use range coder with default table.

Appropriate internal coder type values have been added and used in any
check rather than using raw numbers.

Initialization of avctx.coder_type in ffv1dec is removed because this
field is encoder only. An unneeded validation check in the encoder
is dropped too.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
