D3D: Reintroduce depencency on D3DX11 because it's the most straightforward way to bring back some broken features.

The Dolphin development team is incapable of providing sufficient replacement for its previous usage in Dolphin and the advantages of dropping the dependency do not justify the removal of screenshots and texture dumping.

From now on, d3dx11.h, d3dx11async.h, d3dx11core.h and d3dx11tex.h are required to be stored somewhere in the header include path. I don't know if this is the case for anyone else than me, but I can't really say that I care after having people randomly merge unfinished branches into master.
D3D: Use D3DX for texture dumping.
Revert "Be less dumb."
Revert "Actually, filename really does need to be a parameter because of some random debug thing."
Revert "fix non-HAVE_WX case"
Revert "Handle screenshot saving in RenderBase.  Removes dependency on D3DX11 for screenshots (texture dumping is still broken)."

This reverts commits 00fe5057f13bca3416b9d25a1fe9df27c514b29c, 74b5fb3ab4b066681336d1be44c7eecfcf6a4dc6, cd46138d299088631bbcbf37543584946c4b174a and 5f72542e0637799092ad1006d16ebad1961020e0 because taking screenshots in D3D still crashed for me so there was no point in the code changes (which I found ugly anyway).
D3D: Use D3DX11 for taking screenshots.