Refactor some of texcache for clut render changes

[unknownbrackets]

This does a few things:

1. Moves the scaleFactor check to once per frame; this way we don't have g_Config.iTexScalingLevel == 1 checks that are wrong in some auto modes.
2. Corrects render-to-texture in d3d9, which it turns out I broke entirely in Split texture application into two phases #7870 (depth was out of range.)
3. Refactors out the shading of a separate texture, so the same code can be reused for indexing. No benefits yet, but it works and will reduce the size and complexity of Process rendered CLUTs on the GPU #8246.

Was good that I refactored this as it made me realize change 2 above was needed. Was really confusing me why d3d9 didn't work, because I had the assumption that depal was working.

-[Unknown]

[hrydgard]

This ShaderApplier might be nice if we implement on-GPU texture upscaling as well.