Remove the "gamma_" prefix from "gamma_srgb_to_linear" and "gamma_linear_to_srgb" in color.rs

[echochamber]

Not something I know a lot about, but from reading up on the topic, it seems that the "gamma" prefix in these functions is redundant since it is implied by srgb.

Since they are public functions changing the name would be a breaking change, so it may not be worth the headache it creates for such a trivial change. However, it would make for a nicer API and piston is still pre 1.0.

Link to the code