Add new metal images, add dispersion images, and fix parameter ranges

[peterkutz]

This PR makes a few miscellaneous changes:

• Adds renders comparing several real metals without and with correctly-set specular edge tints. Both the base colors and the edge tints for these renders were derived from real measured spectral data and the full Fresnel equations.
• Adds dispersion images showing the effect of linearly scaling the dispersion across its full default range on a piece of glass.
• Makes parameter ranges closed intervals to allow parameters to be set to exactly their upper bound (1 in most cases, or 3, or 1000). There doesn't seem to be any reason to prevent colors and roughnesses for example from being set to exactly 1.
• Removes periods on one-sentence image captions for consistency and simplicity.

It's likely that better images could be made that show the relevant effects more clearly (without zooming or switching back and forth), but these images should work well as placeholders.

[peterkutz]

Thanks for the quick review @portsmouth .

I just force-pushed to update the image names. No other changes.