[d3d8] Add NVIDIA shadow filtering vendor hack for d3d8

[AlpyneDreams]

Part of #3411, but not a requirement for #3411 to merge.

Features:

• Some early D3D8 games expect Dref to be on the range of [0..2^bitDepth - 1], so this adds a config option for DXSO and D3D9 fixed function to scale it back down to [0..1]. Source
• Certain D3D8 games expect hardware to perform shadow filtering when they sample depth textures. This PR adds another config option to enable a basic 2x2 PCF to emulate hardware shadow filtering.

More info:

• http://www.jiri-dvorak.cz/scellpt/
• Add support for NVIDIA Shadow Buffers AlpyneDreams/d8vk#65
• Splinter Cell AlpyneDreams/d8vk#69

[AlpyneDreams]

Goals:

• Re-implement for DXSO ideally without a spec constant.
• Re-implement in fixed function renderer ideally without extending the PS shader key (want to avoid variants).
  • Use D3D9FixedFunctionOptions if necessary to configure behavior
  • Had to use shader key for now. When fixed function is rewritten it can be checked at runtime.
• Try to avoid conditionals in the pixel shader too
• Should also work with Splinter Cell: Pandora Tomorrow as well as Splinter Cell
  • And be applicable for other games

Options:

• Best case: entirely hard-coded based on texture type and config settings for both FF and DXSO
• Push constants have more room than spec constants
• Extending D3D9SharedPS is also an option

Questions:

• Can the Dref scale factor be hard-coded per-game (e.g. if game only uses D24S8) or per-shader (most likely case assuming games never bind more than one depth texture in a shader anyway)?