PAINTROID-687

Improved the performance of the Zoom Window feature by:

• Removing redundant operations in the code (which are not needed anymore after the fixes in Paintroid-680 and Paintroid-686)
• Changing some of the calculations for the zoom window bitmap making them more efficient.
• Changing the way the zoom window bitmap is getting cropped out from the whole bitmap (by drawing a circle around the coordinates where the user is currently drawing with the desired radius and then cropping it out).

This seemed to make it much faster now, especially on the emulator in Android Studio it was a significant improvement. In my opinion the improvement can also be felt when drawing on a physical device. At least it feels a little bit smoother now on the physical device.

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