better handling of layer mask save during multimask update

[mradamcox]

Saving the multimask was very slow with the recent changes that in caused a save operation on each layer in the multimask. To fix this, this PR

• Only saves layer masks where the mask has actually changed
• Does not update the extent of the layer from the file when the layer save is called
  • Frankly, this should become the default, but much more would need to be done to properly do that