Add support for texture atlases

[ianmackenzie]

Generally make it easier to construct things like quads that refer to a particular subsection of a texture...this has come up a couple times already on the Elm Slack and could be done in a few different ways:

• Have a version of Scene3d.quad (Scene3d.sprite maybe?) that allows specifying a custom UV range
• Have a way to create a Material value from a section of a texture so that mesh UV coordinates can still range from 0 to 1 but will end up referring to a subsection of a texture

The latter is probably nicer (figuring out what sections of a texture atlas correspond to different objects does seem like more of a concern of the texture than the objects) but will require a bit more thought to figure out how it should work.