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.

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.

This fixes various texture offsetting issues with negative texture coordinates (bringing the software renderer in line with the hardware renderers).  It also handles the invalid wrap mode accurately (as was done for the hardware renderers in the previous commit).  Lastly, it handles wrapping with non-power-of-2 texture sizes in a hardware-accurate way (which is somewhat broken looking, as games aren't supposed to use wrapping with non-power-of-2 sizes); this has not been done for the hardware renderers.

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.

Additional changes:
- For TevStageCombiner's ColorCombiner and AlphaCombiner, op/comparison and scale/compare_mode have been split as there are different meanings and enums if bias is set to compare.  (Shift has also been renamed to scale)
- In TexMode0, min_filter has been split into min_mip and min_filter.
- In TexImage1, image_type is now cache_manually_managed.
- The unused bit in GenMode is now exposed.
- LPSize's lineaspect is now named adjust_for_aspect_ratio.