TextureWidget crashes when there are cached video textures

[speigg]

getTextureFromSrc from the TextureWidget assumes that each textureCache hash is simply JSON string, which is not always the case. See: https://github.com/aframevr/aframe/blob/6a236273bfd6bd673e36ff4d376135906ba57ed6/src/systems/material.js#L207

When trying to JSON.parse() a textureCache hash that is generated from a video texture, the TextureWidget throws an "Unexpected token a in JSON" error.

[vincentfretin]

This has been fixed by #680