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.

This more accurately represents what's going on, and also ends at 0 instead of 1, making some indexing operations easier.  This also changes it so that position_matrix_index_cache actually starts from index 0 instead of index 1.

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.

Gets rid of some indirect inclusions in cpp files.
Also this will reduce the amount of rebuilt files if
changes occur in the DataReader header.

Also remedies places where the video backends and core rely on things
being indirectly included.

These functions aren't called from jitted code anymore so it isn't necessary.