WinRT: bug-fix - SDL_SetThreadPriority() didn't work on WinRT 8.x platforms

WinRT 8.0 (Phone and non-Phone) didn't offer an API to set an already-created
thread's priority. WinRT 8.1 offered this API, along with several other
Win32 thread functions that were previously unavailable (in WinRT).

This change makes WinRT 8.1+ platforms use SDL's Win32 backend.

WinRT: minor code cleanup in window-creation code

Window flags can be detected on all WinRT platforms.

WinRT: native windows are inherently resizable (in WinRT 8.x)

WinRT: bug-fix, fullscreen mode wasn't getting reported in Windows 8.0 apps

This bug did not occur in Windows 8.1 apps, just Windows 8.0.

WinRT: bug-fix, OpenGLES2 was failing to init, after recent windowing fixes

Some SDL_Window flags were getting lost inside WINRT_UpdateWindowFlags, namely
SDL_WINDOW_OPENGL.

WinRT: bug-fix, fullscreen window flags weren't set if device was rotated 90 degrees

WinRT: lots of display and windowing related fixes

This change-set fixes a lot of windowing related bugs, especially with
regards to Windows 8.x apps running on Windows 10 (which was the driver for
this work). The primary fixes include:
* listed display modes were wrong, especially when launching apps into a
non-fullscreen space
* reported window flags were often wrong, especially on Windows 10
* fullscreen/windowed mode switches weren't failing (they are not
programmatically possible in Win 8.x apps).

WinRT: fixed crash in SDL_CondWaitTimeout, when using Win10's MSVC runtime

Replaced tabs with spaces in test programs.

Added SDL_HINT_VIDEO_X11_NET_WM_PING to WhatsNew.txt for 2.0.4.