fix: use gl.finish to enforce synchronization
fix: use gl.finish to enforce synchronization

This way we know a scene is finished when control is returned to
javascript and we can run the loop as fast as possible to finish slicing

DanielJoyce/microtome,Microtome/microtome,DanielJoyce/microtome,DanielJoyce/microtome,Microtome/microtome,Microtome/microtome,DanielJoyce/microtome