Prior to this commit, determining the default brush asset used for a
particular mode was based on the object's mode. This is slightly
incorrect for the Image Editor, since it may be in Paint Mode no matter
the underlying object type. To fix this, use the runtime `PaintMode`
enum for determining these default values instead of `eObjectType` and
store the `PaintMode` on `PaintRuntime` for easy access.
Additionally, inside the toolsystem, prevent accidentally unsetting
the default brush by checking for the presence of the asset when
loading an `AssetWeakReference`.
Pull Request: https://projects.blender.org/blender/blender/pulls/144765
6e82e403df