D3D depth computation rewrite, even more cleaned up.

differences to galop1ns and magumagus branch:
- removed the non-standard viewport, it made no difference
- removed clip shader output, it made no difference
- removed refactorings inside VertexShaderGen that have no apparent effect
- removed other vertex_shader_uid_data members that weren't really used
- removed the zbias to -4 force for 0xFFFFFF