python3 compatibility

[rwb27]

several python3 related fixes:
ctype char * now requries bytes (was string)
some calculations now seem to need int()

[rwb27]

I don't quite understand why the calculations need int(), as I don't think that's my code and I'm fairly sure it worked on Python 3 before... However I can't see that int() will do any harm, so may as well leave it in there.

[codeTom]

I don't know why either but it complained that indices must be integers and that (float & int) is not a valid expression without them.

[rwb27]

Fair enough! I wonder if perhaps the cause of the error is elsewhere, perhaps I've done a / rather than a // somewhere else in the code I added, and I didn't notice because I only tested in Python 2. If I get time I might test the upstream picamera module and see if it has the same issue.