unify color specification

colors were a mixture of 4bpp integers (0=white, 15=black) and
fractional blackness levels (0=white, 1.0=black) before. This is
now unified to use the color specification of the Blitbuffer API.
fill background of intermediary blitbuffers

we need to explicitly fill those buffers, now that white is not "0"
anymore.
update to latest base