color-to-alpha: merge back color-to-alpha-plus changes
Merge the color-to-alpha-plus changes back into color-to-alpha, and remove it from the workshop. Remove the compress-threshold-range property of color-to-alpha-plus: keep only the compress-threshold-range == FALSE behavior. Disable the OpenCL implementation, since it doesn't support the new properties. It should be re-enabled once the OpenCL version implements them.
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