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/wl_init.c

@@ -91,9 +91,9 @@ static void pointerHandleMotion(void* data,
         window->wl.cursorPosY = wl_fixed_to_double(sy);
     }
 
-    _glfwInputCursorMotion(window,
-                           wl_fixed_to_double(sx),
-                           wl_fixed_to_double(sy));
+    _glfwInputCursorPos(window,
+                        wl_fixed_to_double(sx),
+                        wl_fixed_to_double(sy));
 }
 
 static void pointerHandleButton(void* data,