Currently, there is no way to call the sculpt.mask_by_color operator
from the python API without simulating mouse events. This makes writing
python tests harder than it needs to be.
This commit does the following:
* Adds an exec callback for the mask_by_color operator.
* Adds new `location` property, an int array defined in region space
coordinates to represent the mouse location.
* Extracts logic into a helper function so that the `invoke` and `exec`
paths are functionally equivalent.
Pull Request: https://projects.blender.org/blender/blender/pulls/134964
