The `DefaultMixer` for mixing generic data types has some issues:
1. The full buffer is always zeroed, even if only some is used.
2. Finalizing also works on all values, even if only some are used.
3. "mixing" doesn't allow setting the first value, requiring that
everything is cleared beforehand.
This commit adds the following functionality:
1. Constructor with the specified `IndexMask` for preliminary zeroing.
2. `set` method to overwrite the value.
3. `finalize` with the specified mask to process a subset of values.
This is useful in situations where you want to use the
DefaultMixer without having to overwrite all the values many times.
A performance improvement was observed for NURBS curve evaluation and
attribute interpolation from the point to curve domain of about 15% and
35% respectively (100,000 curves).
Differential Revision: https://developer.blender.org/D15434
55a332da64