pmcx incorrectly set default source focal length to 1

[fangq]

it appears that this is a severe bug affecting all past pmcx versions - that the default source focal length is 1, which should have been 0 (disabled).

This bug yields incorrect results for all area sources, including fourier, disk, planar and more. It does not affect point sources such as cone/pencil/isotropic.

this was reported in this mailing list post by Gijs Buist:

https://groups.google.com/g/mcx-users/c/SFl6HskLGxU