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/cocoa_platform.h

@@ -99,6 +99,8 @@ typedef struct _GLFWlibraryNS
     short int           publicKeys[256];
     short int           nativeKeys[GLFW_KEY_LAST + 1];
     char*               clipboardString;
+    // Where to place the cursor when re-enabled
+    double              restoreCursorPosX, restoreCursorPosY;
 
     struct {
         CFBundleRef     bundle;