Several improvements and bugfixes on plotting, and starting to make things nogc

[aferust]

looks like more work is needed to get rid of the Image class without breaking the compatibility of too many existing codes. The final goal should be GC-free video loops, such as:

auto fig = imshow(...);
while(readFrames){
    ....
    fig.draw(aNewFrame);
    ....
    waitkey(k);
}