Clear transform after painting each object

Resolves #701.

The transform which is set by the `Shoes::Swt::Common::Painter`, if it's provided by the object, was being allowed to persist across to additional painting operations on that graphics context.

Most art objects didn't actually have a transform set, but line does so elements following it would get mixed up by having the line's transform applied to them.

Should this same pattern apply to the other things we're setting in the painter?
```
       graphics_context.set_antialias ::Swt::SWT::ON
       graphics_context.set_line_cap(LINECAP[@obj.dsl.style[:cap]] || LINECAP[:rect])
```

Also note I didn't write up a spec about this since at this point `Shoes::Swt::Common::Painter#paint_object` isn't really tested at all, and I just didn't have the gumption tonight to start writing that spec. :zzz:

If I feel bad enough I might work that spec up before we merge this, otherwise it might be worth a separate issue for some hearty soul to tackle.