Comment out sampler uniforms in GLSL vertex shaders.
Changed the Direct3D library fallback mechanism: on MinGW the SDK is never searched for, because if it is found, the libraries likely are incompatible. Instead the fallback libraries are set in Urho3D-CMake-magic.cmake if the SDK was not found or not searched for. Documented the use of MinGW-w64 to enable a fully functional Direct3D build.