You can use fully transparent surfaces to restrict movements.
You could also try appending #nobasis hash to your scene address to force the use of non-compressed textures and see how quality changes.
We are thinking of allowing control for texture scaling. There are more and more art galleries done with Shapespark for which such controls would help to improve quality of important art work.
The white dots between the quads can be a result of inaccurate representation of objects position. If each quad has a separate texture it needs to be a separate object. For geometry instances detection purposes and to improve compression of geometry, Shapespark transforms each object to be placed at the [0, 0, 0] origin, and stores a transformation to be applied to the object to move it back to a correct place in a scene. Floating point inaccuracies can cause the objects that perfectly aligned to have a tiny gap between them after such transformation.