Manages the creation of image atlases.
Images added to this manager will be inserted into an atlas, which
will be used for rendering.
size given in the constructor is the size for the first
atlas. After that, when new atlases are created, they will have
twice the size as the latest atlas (until
maxSize is reached).
If an application uses many images or very large images, it is recommended
to set a higher
size value to avoid the creation of too many atlases.