Add SFML example (#133)

* Add SFML example
Adds a rough demo using SFML for rendering.

* Simplifications to SFML example

* Rely on sfml 0.16 rather than a git revision
* Avoid allocating temporary Path
* Take into account margin and spacing
* Directly produce a FloatRect, since it is what SFML needs
* Renamed VertexMesh to QuadMesh
* Use Vertex::with_pos_coords since we always passed Color::WHITE
* Removed TileLayer wrapper, since it added no value
* Removed handling of gid == 0 in Tilesheet::tile_rect

* Update contributors list

* Apply formatting

* Avoid the need to inverse the camera matrix

* De-hardcode the tile size

Also fixes the example a bit, because the map was drawn too small.

* Update workflow

* Try fixing CI issue by removing rust-cache

* Fix CI

* Fix map asset path

* Merge SFML and Rust workflow

* Misc changes

* Merge upstream/master

* Update example to master

* Fix line endings

* Fixed comment

Co-authored-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>