Remove texture atlas support

This patch does a few different things.
- Remove the setting enable_texture_atlas. Remove TextureSource::buildMainAtlas() and code that calls it.
- Remove AtlasPointer. This is the biggest change. Without atlas support, x0 and y0 can be assumed to be 0 and x1 and y1 to be 1. All code that only needs a video::ITexture* just uses that directly now; code that also needs the texture id grabs that from a TileSpec (I noticed that a TileSpec was always available when the texture id of an AtlasPointer was needed).
- Rename SourceAtlasPointer to TextureInfo and remove unneeded fields.
- Replace getTexture/getTextureRaw/getTextureRawAP with a simplified API (two versions of getTexture: one that takes an ID and returns an ITexture*, and another that takes a name and returns an ITexture* and optionally the ID). Remove updateAP. These changes make TextureSource a whole lot easier to understand.
- Remove the ^[forcesingle texture modification. Mods never needed to use this (and hopefully none did) and the core doesn't need it anymore.
- A fix to the TileSpec constructor that removes an uninitialised value warning in valgrind.
- A fix to ~TextureSource that removes irrlicht warnings that used to appear on disconnect ("Could not open file of texture: ...")
- A fix to addNodeParticle: When adding a particle with an animated texture (e.g. when digging a torch), the old code used to not produce weird looking particles merely because of a hidden hack. Now the solution is made explicit (without cluttering up the code or making it slower).

Now "grep -Ri atlas minetest/src/" returns no results except in sqlite and cguittfont :D