Clipping sliceplanes

[alexhuth]

This adds the option to clip the brain surface at sliceplanes. This enables simultaneous viewing of surface and deeper subcortical data. (Although to make this really useful we might need a way to have different datasets visible on the surface and sliceplanes, perhaps?)

It adds a new submenu, clip, under sliceplanes, with boolean toggles for clip_{x,y,z} and flip_{x,y,z}. The clip toggles hide the surface on one side of the sliceplane, and flip toggles switch which side that is. The sliceplanes can be moved and rotated as normal.

Here's a small script that you can run to test (it works better if you use a subject where the brainmask.nii.gz volume is available unlike the default subject S1; in that case replace 'raw' with 'brainmask'):

import numpy as np
import cortex

subject = "S1"
anatdata = cortex.db.get_anat(subject, 'raw').get_fdata().T
anatdata[anatdata < 5] = np.nan
anatvol = cortex.Volume(anatdata, subject, 'identity')

cortex.webshow(anatvol)

I've also attached a screenshot of what it looks like with a subject where the masked brain is available.

I haven't tested how this works with vertex data or RGB data or really anything but what you see. It's a start, anyway :)