This is a clean-up pass that eliminates a few problematic patterns:
* Eliminating redundant parentheses around simple expressions.
* Combing declaration and assignment of variables where appropriate.
* Moving variable declarations closer to their first use.
* Many variables and arguments have been marked as `const`.
* Using `LISTBASE_FOREACH_*` variants where applicable instead of
manually managing loop control flow.
There are no functional changes.
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D16459
5671e7a92c