Align GC behavior on `init` calls on all platforms

### Problem description

You can call `init` on `GC` instance multiple times. We have to align what should happen if you call it for the second time with different dimensions or without any parameters at all.

Currently, iOS uses `0` for the dimension which is undefined and all drawings are removed on each `init` call.

### Expected behavior

We expect that all platforms are consistent :-)

### Environment

- Tabris.js version: 3.0.0-beta
- Device: any
- OS: any