Our composite modes don't correspond directly to the Porter-Duff
operators after which they're named, and these names aren't too
descriptive anyway.

Rename the composite modes as follows:

  Source Over       =>  Union
  Source Atop       =>  Clip to Backdrop
  Destination Atop  =>  Clip to Layer
  Source In         =>  Intersection

Update relevant code, including UI text, enumerator names, function
names, and action names.