Add ImGui_ImplSDL2_InitForD3D()

[josiahmanson]

This is a trivial change to add the function ImGui_ImplSDL2_InitForD3D() to the SDL platform. It can be useful to use SDL with D3D as a rendering backend, because SDL has a variety of other platform interfaces like input handling, etc.

[ocornut]

Hello @josiahmanson ,
Ideally I would like an example_sdl_directxXX/ project that implemented this and also worked in the docking branch with multi-viewports enabled.

Linking to #2612

For multi-viewport, I think we should do something like in this commit:

yaz0r@bbe0eb9

Instead of adding a function I would just add a new field in ImGuiViewport*, e.g. adding void* PlatformHandleRaw next to void *PlatformHandle.

I can handle the design and change for multi-viewports if there is a PR for the basic example working in master.

[yaz0r]

@ocornut Still need to do a proper PR for this. I will try to get that done over the weekend.

[ocornut]

This is now merged as part of #2632, thanks both!