Fixes LIT (https://bugs.dolphin-emu.org/issues/13635). The text does not include normals, but has lighting enabled. With the previous default of (0, 0, 0), lighting was always black (as dot(X, (0, 0, 0)) is always 0). It seems like the normal from the map in the background (0, 0, 1) is re-used.
LIT also has the vertex color enabled while vertex color is not specified, the same as SMS's debug cubes; the default MissingColorValue GameINI value of solid white seems to work correctly in this case.
This adjusts the NaN replacement logic introduced in #9928 to work around the HLSL compiler optimizing away calls to isnan, which caused that functionality to not work with ubershaders on D3D11 and D3D12 (it did work with specialized shaders, despite a warning being logged for both; that warning is also now gone). Note that the `D3DCOMPILE_IEEE_STRICTNESS` flag did not solve this issue, despite the warning suggesting that it might.
Suggested by @kayru and @jamiehayes.
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.
