adjustable and single channel shaders

[ClaireBookworm]

added adjustable shader which can be super helpful for viewing. can be nice to also add a shader that loads distances and more complex things too.

Possible notes:
the single channel shader is still really jank—once you manually set the channel to your -c [x] channel, the shader works perfectly. but I'm not quite sure how to auto-load the channel (and only that channel) without seemingly having to load the entire file at once (i.e., doing a.data[channel:channel+1]). Therefore, the shader code just gives you the normalization curve of the channel you inputted, which means if you use a diff channel than your flag, it won't update.