save time: pre initialize and load GPU

[mafrieger]

maybe its only because the feature GPU rendering is cool new and this leads to often having to try it,
but it would be handy to to reduce waiting times when switching first time to GPU by prepraring everything needed to start GPU-Rendering in background: from GPU initialisation to GPU materials

Of course this may generate a little aditional system load, so one could do this if system load is low and/or add a a setting to do this in background on startup.