[d3d8] Nvidia Shadow Buffers support

[WinterSnowfall]

Just so we don't forget about it. Nvidia Shadow Buffers (a cutting edge GeForce 4 tech at the time, whose marketing claims remind one of modern age ray tracing) are explicitly needed in Splinter Cell: Pandora Tomorrow, but also enhance the experience in the first game of the series.

@AlpyneDreams had a d8vk branch that added support (d9vk-level changes were also involved IIRC), but it never made it as part of the d8vk merge, apart from a dxvk.conf/config.cpp remnant that got removed in #4144 . We should revisit the topic at some point for sure.