Fixed black mipmap levels in DXT textures (DirectX) - Issue #1783.

This pull request fixes a bug were the last two mipmap levels of DXT textures are black in DirectX. See also Issue #1783.
It also adds support for SamplerState.MipMapLevelOfDetailBias in OpenGL (which I used for testing).

This was tested on Windows desktop (DirectX and OpenGL) and Windows Store.
We have tested several textures types: 
- Color/DXT1/DXT5
- power-of-two vs. non-power-of-two
- DXT and mipmaps generated by XNA vs. DXT and mipmaps generated by the DirectX Texture Tool

--> No more DXT problems found in DirectX.