Implements the design from #116067.
The socket type is called "Matrix" but it is often referred to as "Transform"
when that's what it is semantically. The attribute type is "4x4 Matrix" since
that's a lower level choice. Currently matrix sockets are always passed
around internally as `float4x4`, but that can be optimized in the future
when smaller types would give the same behavior.
A new "Matrix" utilities category has the following set of initial nodes"
- **Combine Transform**
- **Separate Transform**
- **Multiply Matrices**
- **Transform Direction**
- **Transform Vector**
- **Invert Matrix**
- **Transpose Matrix**
The nodes and socket type are behind an experimental flag for now,
which will give us time to make sure it's the right set of initial nodes.
The viewer node overlay doesn't support matrices-- they aren't supported
for rendering in general. They also aren't supported in the modifier interface
currently. But they are supported in the spreadsheet, where the value is
displayed in a tooltip.
Pull Request: https://projects.blender.org/blender/blender/pulls/116166
1cfe9dd08c