This worked correctly on the JIT vertex loaders, and for the equivalent case with texture coordinates. I'm not aware of any games this affects (but libogc does mention a semi-related scenario at https://github.com/devkitPro/libogc/blob/6bc0317c7d52063c2d2b48b721520093c76432a9/gc/ogc/gx.h#L1855-L1857.)

This regressed in 0a906f553fe7699d1c863a464211f74b6c306a0f, I think (though I haven't confirmed it). Mario Tennis and Luigi's Mansion both use these for some reason (as far as I can tell, the data isn't actually used; it's just extra data included for no reason)

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.