Convert any inventory item into a mesh, bring back InventoryItem::getImageRay(), some const-correctness fixes

Made wielded tool move slightly (and smoothly) during view bobbing. Making the tool be a child node of an empty scene node instead of the camera scene node seemingly fixed the uncontrollable tool jitter, too.

Collected and moved existing camera infrastructure from game.cpp to camera.cpp and camera.h. Introduced configuration settings 'fov' which chooses the camera's (vertical) field of view and 'view_bobbing' which currently does nothing. Other code refactored to not expect the FOV to be a build time constant.

Removed unused camera_position and camera_direction fields from Client. Moved ClientEnvironment::drawPostFx to ClientMap::renderPostFx -- this will make the camera management classes easier to write, as ClientMap already knows the camera position but ClientEnvironment doesn't and has to be told about it. This also eliminates the need for Client::getEnv(). Made the post effect color a content feature defined in content_mapnode.h.

And some simple animation when trying to dig...

Speed up periodical map unloading by moving beginSave() and endSave() to outer loop in Map::timerUpdate()

Fix some texture stuff: remove item_fence.png, use fence.png, remove inexisting cactus.png from texture atlas (replace with nothing because cactus is rare)

Also, the temp item wasn't being deleted, might have been a memory leak.
Now you will only eat 1 item off a stack

* The map will reside in world/map.sqlite
* It will load from the sectors folder but will not save there

* I didn't understand that expression last fix and now the furnace would burn fuel
even when it had no source item.