Use a display buffer for rendering

Render to a background texture, and only show the context of this texture when presenting the
Hoder tileset: set background tile "2" of the additional tiles of t_grass.

The grass foreground tile has transparent parts and those parts show the content of the previous frame in game.
Instead they should show the dirt under the grass (dirt has tile "2" and tile "2" is completely opaque).
Store previously_pressed_key in sdl version, too.
Merge remote-tracking branch 'BevapDin/sdl-butcher' into FrankenMod
Merge remote-tracking branch 'BevapDin/sdl-fixes' into FrankenMod
Item_factory: add support for loading items of type it_stationary.

- Simplify the constructor of it_stationary to make it usable in Item_factory (all the values of it_type are loaded _after_ the object has been created).
Export the flyer item to json.
Allow snippet texts to be read from an array

instead of having each entry in a separate json object.
Add documentation for json type STATIONARY and snippet.
Add extraction of translations for stationary items

- add starting locations to translatable entries to make the lang/extract_json_strings.py working
Merge remote-tracking branch 'BevapDin/stationary' into FrankenMod