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.
(Specifically, the copy for VertexLoaderManager::position_cache. The position matrix index happens elsewhere, and the float path still has special logic to copy to scratch3.)
This increases accuracy, fixing the white rendering in Major Minor's Majestic March. However, the hardware backends can only have one viewport and scissor rectangle at a time, while sometimes multiple are needed to accurately emulate what is happening. If possible, this will need to be fixed later.
I think this is a relic of D3D9. D3D11 and D3D12 seem to work fine without it. Plus, ViewportCorrectionMatrix just didn't work correctly (at least with the viewports being generated by the new scissor code).
These aren't particularly useful, and make the code a bit more confusing. If for some reason someone wants to test what happens when these functions are disabled, it's easier to just edit the code that implements them. They aren't exposed in the UI, so one would need to restart Dolphin to do it anyways.
JMC47andGitHub
Pokechu22andScott Mansell
OatmealDome
Admiral H. Curtiss
JosJuice
Mai MandGitHub