This commit changes the internal storage and user-facing representation
of the brush size values (`size` and `unprojected_size`) from radius
to diameter.
This has a number of benefits:
* While the radius is more helpful for many internal operations, it is
more natural to estimate the size of a brush by the diameter
* Because the pixel size is stored as an integer, users are currently
unable to make brushes that have an odd numbered diameter, notably
preventing the ability to make single pixel brushes.
Internally, the `Brush` and `UnifiedPaintSettings` size values are
versioned to double their on-disk values. The relevant `BKE` functions
that access the data return the radius at runtime, and any internal
`struct`s that stored radius continue to do so.
The 'Radius' text for brushes is changed to 'Size' and all references
to it in descriptions are changed to 'size' as well.
Resolves#134204
Pull Request: https://projects.blender.org/blender/blender/pulls/142495
8188dbd246