Move all cursor positioning to platform code

Due to Wayland, shared code cannot rely on cursor positioning being
supported by the underlying platform.

This implicitly fixes #617 as it moves cursor centering into
_glfwPlatformSetCursorMode, thus separating it from the stale value of
_glfw.cursorWindow.

Fixes #617.

src/internal.h

@@ -437,9 +437,6 @@ struct _GLFWlibrary
         int             refreshRate;
     } hints;
 
-    // Where to place the cursor when re-enabled
-    double              restoreCursorPosX, restoreCursorPosY;
-
     _GLFWcursor*        cursorListHead;
 
     _GLFWwindow*        windowListHead;
@@ -906,7 +903,7 @@ void _glfwInputMouseClick(_GLFWwindow* window, int button, int action, int mods)
  *  of the client area of the window.
  *  @ingroup event
  */
-void _glfwInputCursorMotion(_GLFWwindow* window, double x, double y);
+void _glfwInputCursorPos(_GLFWwindow* window, double x, double y);
 
 /*! @brief Notifies shared code of a cursor enter/leave event.
  *  @param[in] window The window that received the event.