Blender Git Loki

BLI: remove implicit casts between some span types

Casting pointers from one type to another does change the
value of the pointer in some cases. Therefore, casting a span
that contains pointers of one type to a span that contains
pointers of another type, is not generally safe. In practice, this
issue mainly comes up when dealing with classes that have a
vtable.

There are some special cases that are still allowed. For example,
adding const to the pointer does not change the address.
Also, casting to a void pointer is fine.

In cases where implicit conversion is disabled, but one is sure
that the cast is valid, an explicit call of `span.cast<NewType>()`
can be used.