#12 AVI width/height does not have to be a multiple of 16

[Pa3PyX]

FlaskMPEG enforces width and height of the output to be
a multiple of 16. AVI files have no such restrictions per
se; FlaskMPEG should check the selected encoder for
size and colorspace limitations (a-la VirtualDub).

Besides, Resizing is done first and Cropping later. I think
this is wrong (should be the other way around), but it is a
matter of taste more than anything. However, in any
case, it is not necessary to enforce dimension
restrictions on both operations (as it is done now) -- only
on the one done last, which becomes the input to the
encoder.