QPainter does not like to draw in an empty QImage.
And the rendering code does not like to render it.
So we would get this on the console:
```
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::save: Painter not active
QPainter::save: Painter not active
QPainter::setRenderHint: Painter must be active to set rendering hints
QPainter::save: Painter not active
QPainter::setClipRegion: Painter not active
QPainter::setPen: Painter not active
QPainter::setBrush: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::setPen: Painter not active
QPainter::setBrush: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::save: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::restore: Unbalanced save/restore
thread 'main' panicked at 'attempt to subtract with overflow', sixtyfps_runtime/rendering_backends/gl/texture.rs:285:42
```
Currently the Qt backend still redirect everything to the GL backend,
but the goal is to use QPainter and QWindow
This also adds a "default" backend, whose goal is to select the proper
backend at compile time
