lavc/snowenc: Expose an option to set the rc_eq expression

lavc/snowenc: Expose an option to set the rc_eq expression

Snow uses the ratecontrol module, but does not expose a way to set
the rc_eq expression. The default expression, set in the ratecontrol
module, will always be used.

Make it possible to set rc_eq by adding an AVOption to snowenc.

The option definition is mostly a copy from the mpegvideo common
options definition of rc_eq (libavcodec/mpegvideo.h), with some
minor style adjustments to be closer to the other snowenc option
initializer expressions.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>