Add 'fade' parameter

Usage: tmaskcleaner( length=100, fade=200 ) will zero areas below 100,
assign 0..255 to areas in [length,length+fade] range, and 255 to areas
above length+fade.

This allows adaptive processing of area ranges for example by
transforming the mask from / to /\ with mt_lut("x 128 > 255 x - x ?") or
_||_ to grab some arbitrary range with mt_lut("x 118 > x 138 < | 255 0
?") or to make a non-linear _/\_ mask by smoothly amplifying the middle
with mt_lut("x 128 > 255 x - x ? 100 - 28 / 0.5 ^ 256 *")
Add Y8 to the error message, cosmetics
Merge remote-tracking branch 'tophf/fade'
Update avisynth.h to a5
Minor refactorings, better consistency with other plugins
Error on negative fade