Material dithering

[Firepal]

Description:
Made material dithering available from the component and enabled by default as it is much more preferable than without.

Here's two images of a lit cube seen from above, with contrast edited in-post to show the difference dithering makes.

[dmarcos]

Thanks. We need to document the new property: https://github.com/aframevr/aframe/blob/master/docs/components/material.md

[Firepal]

@dmarcos donesies

[dmarcos]

Thanks!