Optimize map rendering

[lumiscosity]

Right now, the map is always fully redrawn every time something as minute as placing a tile happens. It also doesn't store information between scroll states, opting to fully redraw the map. It would be wise to draw the entire map to a full pixmap and keep it in memory, only redrawing the parts that need to be redrawn. QGraphicsViews should already have built-in scrollbars, there's no need to reimplement scrolling...

Down the line, it could be interesting to provide an alternative QRhi backend. Right now the API is fairly experimental, but we could stand to gain a lot of performance improvements from hardware rendering.

[lumiscosity]

Did some testing to see how viable this is: this does in fact bring large performance benefits for scrolling, but it also makes map loading and tile placement way more expensive. A hybrid approach might be necessary, where the map is rendered on the fly in 8x8 or so chunks; i'd imagine we could also use these chunks to speed up event-based calculations.

[lumiscosity]

Progress will be contained in the render_improvements branch for now; we're still focusing on the roadmap.