rename MutPixels to PixelsMut
rename MutPixels to PixelsMut

This follows the rename of `slice::MutChunks` into `slice::ChunksMut` to adhere
to naming conventions.

hetzge/image,PistonDevelopers/image,twoodford/image,huonw/image,mbrubeck/image,TyOverby/rust-image,PistonDevelopers/image,ruud-v-a/image,lady-segfault/image,theotherphil/image,nwin/rust-image,bvssvni/image,JaySon-Huang/image,tomaka/image,dtwood/piston-image