Blender Git Loki

Compile BKE icons source file with C++

That way we can use scoped mutex locks, which should make code cleaner/saver
and avoids the annoying unlock calls for every early-exit. Will happen in a
followup commit.

Mostly had to address casts from `void *` of course. Used C style casts, almost
all of the file still uses C style coding anyway.
`BKE_icon_geom_from_memory()` was using `goto`s for cleanup logic which
wouldn't compile with C++. Changed to RAII based resource handling.