rendering multiple rasters

[freeman-lab]

We need to do some testing and maybe make some tweaks for the use case of multiple Raster components rendered simultaneously. It should be easy to turn layers separately on and off via the display prop it should be, but there are at least two things we need to improve:

• Preventing data fetching when a layer is set to display=false (probably by skipping updateCamera at either the tile or Raster level)
• Transparency blending when multiple overlapping Rasters have display=true

I believe the way we're using premultiplied alpha will produce a nice blending effect, but needs to be confirmed. The data fetching thing is definitely a current (undesirable) behavior that just needs to be addressed.

[katamartin]

Seems like we might be able to close this after making updates in #26! We didn't implement the first bullet, but the same behavior should be supported now by conditionally rendering a Raster. Thoughts @freeman-lab?