Make the JPEG/Zen mask tolerant of pure colors in band interleaved MRFs

[lucianpls]

The current behavior is that a zero on any band on input will result on zeros on all bands on output, since there is only one zero mask. This leads to some saturated colors pixels in input to be converted to black.
Instead, the default behavior should be to only flag as black pixels that have at least one zero value in a band and relatively small values in the other bands (very dark pixels). This would work better for most cases.
The original behavior (any zero masks the whole pixel) could be removed or made available via a JPEG specific create option.

[lucianpls]

This was done a while ago, the mask only flags the pixel as zero if all the bands are zero.