[aurorae] - support blurregion from kdecoration2

This is an approach for aurorae engine to publish masks for its decorated windows in order to avoid out of window blurring at the decoration corners. Aurorae themes are now able to specify a **mask** element inside **decoration.svg** file like plasma themes already do. Mask is used afterwards to calculate theme's blur region.


| Before | After |
| ------ | ----- |
|![before](/uploads/26014e79c3d5d45ba12fa5cf62294b1c/before.png)|![after](/uploads/923d7021eaaf322be96b611c73558666/after.png)|

Adjusted Aurorae theme for testing: [ROUNDED-DARK.tar.gz](/uploads/082f60ad4311e3e296b7faeeb7c97dac/ROUNDED-DARK.tar.gz)

BUG:395725