UI: Use rect intersection test for validating position

Instead of checking to see if the window's position is valid, check to
see if the rectangles of the window and the monitor intersect via a
rectangle intersection test.

UI/obs-app.hpp

@@ -166,7 +166,7 @@ inline const char *Str(const char *lookup) {return App()->GetString(lookup);}
 bool GetFileSafeName(const char *name, std::string &file);
 bool GetClosestUnusedFileName(std::string &path, const char *extension);
 
-bool WindowPositionValid(int x, int y);
+bool WindowPositionValid(QRect rect);
 
 static inline int GetProfilePath(char *path, size_t size, const char *file)
 {