Added methods that can be used to temporarily change the channel mask of an image. (#777)
Added methods that can be used to temporarily change the channel mask of an image. (#777)

In ImageMagick 6 there are "overloads" for methods to only work on the specified channel. Those methods are no longer available in ImageMagick 7. To only change a specific channel a mask needs to be set. This PR adds some macros to make it easier to temporary change the channel mask.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick