Fixed PlotCanvas bug where a call to the draw method might occur before the first Size event is triggered and hence, it has no buffer yet
gui: freeze/thaw redisplay while swapping viewports

will hopefully reduce flickering
Merge branch 'master' of vpn.delmic.com:odemis
gui canvas: beginning of clean up of PlotCanvas

still not clear what's the problem with _dc_buffer, but it seems that keeping a bmp_buffer after
a resize messes up everything.