Convert from sRGB to linear in color picker

[sgmq0]

This addresses issue #1209

Let me know if I'm missing anything, I'm not very sure about the naming conventions.

[jstone-lucasfilm]

This looks like a great start, thanks @sgmq0!

Here are two initial thoughts and recommendations for next steps:

• We should probably add the reverse transform from linear shader colors to the sRGB-encoded space of the color picker, so that the colors that artists see displayed in the viewer interface are in their correct space.
• It's fine to start with just the MaterialX Viewer for this initial pull request, though ultimately we want to extend this change to the MaterialX Graph Editor as well!

[kwokcb]

Checking with nanogui (Viewer) and imgui (Editor) both appear to assume sRGB for display colors so looks like the reverse transform is required.

[jstone-lucasfilm]

I wanted to follow up on this proposal, @sgmq0, to see if you might have the bandwidth to address the requests above. Let us know either way!

[sgmq0]

School got busy and I ended up not having the bandwidth to finish in a timely manner, but thank you for the reminder! I can try to address both requests within a week or so.