D3D: Fixed D3D validation error during EFB to texture copy

Texture was being bound as a render target while still being set as a shader resource.
D3D automatically unbinds the SRV in this case and generates a validation error.
The fix is to manually unbind SRV, render into it and then re-bind to old slots.
VideoSW: rewrite VertexLoader to use the VideoCommon one
D3D: Moved setting texture by slot mask into StateManager
Merge pull request #1689 from kayru/d3d_efb_copy_fix

D3D: Fixed D3D validation error during EFB to texture copy
Merge pull request #1669 from degasus/swvertexloader

Video Software Vertex Loader