- Now each instance of Tileset saves its vertex attributes in a TilesetsBuffer referenced by a static pointer.
- Removed attribute Sprite::vao (the VAO for tilesets is now specified once in the TilesetsBuffer).
- In order to avoid multiple VAO and VBO bindings in each frame, the macro DONT_BIND_WHEN_DRAWING is added. When this macro is NOT defined, the TilesetsBuffer::draw() method performs a first step binding the current VAO and VBO as the active ones.