Sometimes integers are mixed using float weights. In those cases
the mixed result has to be converted from into float again.
Previously, this was done using a simple cast, which was unexpected
because e.g. 14.999 would be cast to 14 instead of 15.
Now, the values are rounded properly.
This can affect existing files unfortunately without a good option
for versioning. Gladly, very few files seem to depend on the details
of the old behavior.
Differential Revision: https://developer.blender.org/D14892
2e70af5cd5