DataReader is generally jank - it has a start and end pointer, but the end pointer is generally not used, and all of the vertex loaders mostly bypassed it anyways.

Wrapper code (the vertex loaer test, as well as Fifo.cpp and OpcodeDecoding.cpp) still uses it, as does the software vertex loader (which is not a subclass of VertexLoader). These can probably be eliminated later.

Fixes https://bugs.dolphin-emu.org/issues/12952

This also means that both a register and a vertex are always specified, though right now if the register is scratch1_reg the offset is always 0.

This way it more closely matches VertexLoaderX64, and is in general easier to understand.

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.

It is no longer relevant for the current set of loaders after 70305425462843af192489da00389559cd6f5834.  If it becomes relevant again, a static function named IsUsable or IsCompatibleWithCurrentMachine or something would be a better approach.

Additionally, VCacheEnhance has been added to UVAT_group1.  According to YAGCD, this field is always 1.

TVtxDesc also now has separate low and high fields whose hex values correspond with the proper registers, instead of having one 33-bit value.  This change was made in a way that should be backwards-compatible.