Implement IndexMut trait for Pixel implementations
Implement IndexMut trait for Pixel implementations

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