[BUG] volume rendering: sigma_clip normalization

[chrishavlin]

This closes #3979 and fixes an unreported bug in which not providing a sigma_clip value while trying to save a non-png results in a crash.

There were several spots where sigma_clip was used to calculated a clipping value, the version in save_annotated incorrectly included the alpha channel in the calculation. I added a ImageArray._clipping_value method to cut down on some of the duplicate code.

Here's a script based on the cookbook example mentioned in #3979

import yt
ds = yt.load("Enzo_64/DD0043/data0043")
sc = yt.create_scene(ds, lens_type="perspective")
source = sc[0]
source.set_field(("gas", "density"))
source.set_log(True)
# Set up the camera parameters: focus, width, resolution, and image orientation
sc.camera.focus = ds.domain_center
sc.camera.resolution = 800
sc.camera.north_vector = [0, 0, 1]
sc.camera.position = [1.7, 1.7, 1.7]
sc.annotate_axes(alpha=0.02)
sc.annotate_domain(ds, color=[1, 1, 1, 0.01])
text_string = f"T = {float(ds.current_time.to('Gyr'))} Gyr"

sc.save_annotated("vol_annotated_6.png", sigma_clip=6, text_annotate=[[(0.1, 0.95), text_string]])
sc.save("vol_save_6.png", sigma_clip=6, render=False)
sc.save("vol_save_6.jpg", sigma_clip=6, render=False)
sc.save("vol_save.jpg", render=False)  # this one errors on main!

The save_annotated and save versions now match:

[chrishavlin]

just a note to reviewers: this was the source of the bug. It should only normalize based on the RGB channels (i.e., im[:,:,:3])

[zingale]

awesome. Nice catch. I never got around to digging deeper, so I'll happily review your PR ;)

[zingale]

I confirmed the output that this works as expected. Nice work @chrishavlin !!

[neutrinoceros]

Nice job ! Found one caveat that can be fixed easily. I’ll merge when it’s dealt with