- Manually check over all direct calls to operator callbacks
ensuring the result isn't assigned to an int.
- OPERATOR_RETVAL_CHECK() now fails unless a wmOperatorStatus is used.
- Check the return values of direct calls to callbacks.
- Remove invalid check for the return value of rna_operator_check_cb.
- Use the variable name `retval` as it's most widely used.
- Move the assignment of `retval` out of the `if` statement in
sculpt/paint operators because it prevents assigning the result
`const` variable.
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