Give proper error message if camera autofocus fails
Setup camera properly before applying camera options

Some of the camera command-line options depend on the aspect-ratio, so
make sure that's correctly set before applying them.
Correctly implement -s/--size option with a single value

When the -s/--size option is given a single value, setting the final
output image dimensions has to be done after the scene has been
loaded, as the value specified should _modify_ any output image
dimensions set by the scene, rather than overriding them based on the
initial default aspect ratio (as was done previously).