Added bounds check in <sprite>:set_texture().
Replaced ugly bool operator in texture classes.
Disable back-face culling.
Deprecated Renderer::draw_elements().
Drawable::update() should take delta time as parameter.
Added getters for width and height of TextureAtlas.
Added Renderer::draw_arrays().
Added support for Spine.
Added Lua::call() to replace a lua_pcall() pattern.