DiffuseMapMaterial ignores the alpha channel

[mrmattf]

Use a png with alpha info for the texture on the diffusemap material. When it's displayed on Cesium, the image has no transparency.

Here's one I'm having issues with.
http://ccmc.gsfc.nasa.gov/ccmc-google_earth/FokRC_e-_precip.png

[IanLilleyT]

Fixed on the visualquality branch with the rgba Image material. DiffuseMap material is still only rgb. Changes will be in master soon.

[pjcozzi]

@IanLilleyT don't close this until we're in master.

[mramato]

@IanLilleyT I assume this can be closed now?