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/win32_platform.h

@@ -252,6 +252,8 @@ typedef struct _GLFWlibraryWin32
     char                keyName[64];
     short int           publicKeys[512];
     short int           nativeKeys[GLFW_KEY_LAST + 1];
+    // Where to place the cursor when re-enabled
+    double              restoreCursorPosX, restoreCursorPosY;
 
     struct {
         HINSTANCE       instance;