Refactor cursor mode paths

This is the refactoring part of adding GLFW_CURSOR_CAPTURED, separated
out to help keep 3.3-stable similar to the main branch.

Related to #58.

src/win32_platform.h

@@ -452,6 +452,8 @@ typedef struct _GLFWlibraryWin32
     double              restoreCursorPosX, restoreCursorPosY;
     // The window whose disabled cursor mode is active
     _GLFWwindow*        disabledCursorWindow;
+    // The window the cursor is captured in
+    _GLFWwindow*        capturedCursorWindow;
     RAWINPUT*           rawInput;
     int                 rawInputSize;
     UINT                mouseTrailSize;