add sRGB encoding to perspective screenshot

[kfarr]

Description: add sRGB encoding to perspective screenshot, fixing #4817

[dmarcos]

I fixed the broken tests on master. If you rebase this is ready to merge. Thanks so much for the patience.

[kfarr]

rebased and no worries, I'm always learning new things working with you and the community!

[dmarcos]

Thank you!

[diarmidmackenzie]

As per a thread in the slack community, I'm concerned that this fix has had a bad effect on colors in some screenshots.

This is what a screenshot of teh HelloWebVR page looked like in 1.2.0.

This is what it looks like in 1.3.0.

This glitch shows that if you revert the changes made in this PR, you get back to the previous coloring (i.e. this change is the cause of the difference).
https://glitch.com/edit/#!/sly-sixth-topaz?path=index.html%3A19%3A9

I'll raise a separate issue to track this, but thought it made sense to flag here as well.

[kfarr]

Thanks, replied here: #5014