Alpha channel support, don't use inventory cubes

Add alpha channel support for latest git clients/servers
(should still work fine on old clients/servers too).

Requires that the client use shaders to see the effect.

Also, removes the calls that create rendered inventory cubes - those
calls are obsolete and should not be used in production, as they prevent
the engine from rendering an actual cube when an item is dropped.
removed all colors from creative inventory in prep for spray booth mod