Auto show darknet predictions slow

[ohlr]

I think the feature auto show darknet predictions only should evoke darknet if there are no marks already.

Going through already marked up images with auto show on is significantly slower then having never show enabled.

Thus I assume darknet gets invoked in the background for no reason.

[stephanecharette]

This is particularly noticeable on CPU (vs GPU) when using large images and tiling has been enabled.

Would be nice to revisit how this part of the code works and skip the calls into Darknet when not necessary.