D3D11 backend appears to hard-depend on SDL in code, but not in build system

[rpavlik]

RenderManagerD3D.h includes SDL.h (for a pointer), and forward-declaring the pointer type doesn't let you build without that include. However, the build system appears to explicitly distinguish between building D3D backend with SDL2 and without SDL2, implying you can actually build without SDL2.

If we can't build the D3D backend without SDL2, then the build system should change so that the requirement for SDL2 is made explicit (rather than becoming a build-time include error). If we should be able to build without SDL2, then there is some code that needs changing.

[russell-taylor]

Related to #29