D3D: Replaced shader-based depth range remap with viewport

This fixes UI rendering in some games mentioned in https://code.google.com/p/dolphin-emu/issues/detail?id=7785
D3D: Removed somewhat mysterious comment

It would be good to know which games exactly exhibited the issue.
Renamed DEPTHPARAMS to PIXELCENTERCORRECTION

This shader constant was previously used for depth remapping in D3D and for pixel center correction. Now it only serves one purpose and the new name makes it clear.
D3D: Viewport min and max depth is now clamped to [0..1] range
Merge pull request #1392 from kayru/d3d_viewport_depth

D3D: Replaced shader-based depth range remap with viewport