Changed drag area API to a hit-testing API.

There were several good arguments for this: it's how Windows works with
 WM_NCHITTEST, SDL doesn't need to manage a list of rects, it allows more
 control over the regions (how do you use rects to cleanly surround a circular
 button?), the callback can be more optimized than a iterating a list of
 rects, and you don't have to send an updated list of rects whenever the
 window resizes or layout changes.

src/dynapi/SDL_dynapi_overrides.h

@@ -580,4 +580,4 @@
 #define SDL_WinRTGetFSPathUTF8 SDL_WinRTGetFSPathUTF8_REAL
 #define SDL_WinRTRunApp SDL_WinRTRunApp_REAL
 #define SDL_CaptureMouse SDL_CaptureMouse_REAL
-#define SDL_SetWindowDragAreas SDL_SetWindowDragAreas_REAL
+#define SDL_SetWindowHitTest SDL_SetWindowHitTest_REAL