Using the `-Wnrvo` flag introduced by GCC 14, I identified a few places where NRVO was clearly intended, but is fumbled.

Now only VertexLoader remains... But that one might be tricky.

To ensure memory safety, callers of GetPointer have to perform a bounds
check. But how is this bounds check supposed to be performed?
GetPointerForRange contained one implementation of a bounds check, but
it was cumbersome, and it also isn't obvious why it's correct.

To make doing the right thing easier, this commit changes GetPointer to
return a span that tells the caller how many bytes it's allowed to
access.

Gets this out of the global namespace.

m_stage is initialized last

This was added in 0b9a72a62d38481ff08f742b2318d07f29ff5dff but became irrelevant in 70f9fc4e7526fc9cfc008a43cd33229f62be99b6 as the check is now self-explanatory due to a rejiggering of the bitfields.

Currently the logic for addressing the individual TexUnits is splattered all
across dolphin's codebase, this commit attempts to consolidate it all into a
single place and formalise it using our new TexUnitAddress struct.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.