This change removes some code that attempted to allow non-standard window sizes, the idea of which was to do display scaling, and a hackish one at that. If display scaling is needed, use SDL_Renderer as appropriate.
- moved SDL_WinRTApp.* from src/video/windowsrt/ to src/core/winrt/, and renamed them to SDL_winrtapp.* (to mimick case-sensitivity used elsewhere in SDL)
- renamed all "windowsrt" directories (in src) to "winrt", as the shorthand name is used more often (and, IMO, "WinRT" != "Windows RT", not entirely at least)