xsystem4.scene: returns a list of scene entities
* all entities have an entityId that can be used in further requests
* SACT2 sprites have a 'sprite' member with a spriteId that can be
used in further requests
xsystem4.renderEntity: renders a scene entity and returns the texture
xsystem4.spriteTexture: returns the stored SACT2 sprite texture
xsystem4.renderParts: renders a parts object and returns the texture
xsystem4.partsTexture: returns the stored parts object texture
Image data in texture objects is RGB8888 encoded in base 64.
