fix: fix Lighting.hlsl warnings

[Pentalimbed]

part of #534

[alandtse]

Did you build with disk cache to find all warnings? I'm seeing the following (but maybe it's VR only).

E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(1956,3-90): warning X3206: implicit truncation of vector type
E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(2355,38-85): warning X3571: pow(f, e) will not work for negative f, use abs(f) or conditionally handle negative values if you expect them

and also

E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(1918,22-124): warning X3206: 'ScreenSpaceShadows::GetScreenSpaceShadow': implicit truncation of vector type
E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(1956,3-90): warning X3206: implicit truncation of vector type
E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(2355,38-85): warning X3571: pow(f, e) will not work for negative f, use abs(f) or conditionally handle negative values if you expect them
E:\SteamLibrary\steamapps\common\SkyrimVR\Data\Shaders\Lighting.hlsl(1008,14): warning X3578: Output value 'main' is not completely initialized

I'm running just this PR so maybe you fixed it with #642?

Also, I didn't test it in game specifically for the effect but I'm ok with solving that later too.

[Pentalimbed]

One of them is fixed by #642 yes

As for pow, I want to fix all of them together by replacing them with a safe pow function, but I don't know where best to put the definition.