Move all the DirectX-specific stuff into a folder.

This actually allows us to compile plPipeline on *nix (although linking
will fail).
Fix clang warnings in plCubicRenderTargetModifier.
plViewTransform is not in plPipeline.

Thanks @Deledrius.
Minor fixes.
Clean up source groups for plDXPipeline.
Actually use working backslashes.
Merge pull request #345 from dpogue/pipeline

Split DirectX stuff into a subdirectory of plPipeline