threshold method improvement

hi matthias,

happy new year to you, i hope you're fine

i' m back at work and made some changes in your pix_depth2rgba object today to suit my needs

i realize that lo_thresh method was not documented and not used

i've just make it possible to pass only value in a range
that way we can extract objects that are at a certain distance

i would like to modify the color assignment according to the range but I don't understand the 1530 value
anyway this is not really important and it works as is (at least for me)

fill free to merge it or not

cheers

antoine