Set a factor for the resulting size in the scale filter

This patch is related to https://trac.ffmpeg.org/ticket/309 and implements the suggestion in the second-last comment.

The problem is basically that if you use the scale filter like ```-filter:v scale=-1:100``` and the size was something like 202:200 before, the resulting width would be 101 pixels which can be a problem for some encoding options. Instead you might want to have results divisible by 2 (or any other factor).

This patch enables such factors by enlaging the accepted input to values lesser than -1, making sure that when a value -n with n > 1 is set, that the resulting width/height of the scale filter is divisible by n.

To take up the example depicted above, ```-filter:v scale=-2:100``` would result in the output video having a width of 100 pixels.