Specifying a file to open at the command line forces zoom to 100%

I'm running a copy of rx installed with `cargo install --git https://github.com/cloudhead/rx --tag v0.4.0` on Kubuntu Linux 16.04 LTS (64-bit) and I observe the following undesirable behaviour.

With the following in `.rxrc` in the current directory...

```
-- Set Cavernas tile size
f/resize 8 8
zoom 8.0

-- Work around the apparent inability to display "actual in-game size" and
-- "preferred editing size" simultaneously
map / :zoom 4.0
map m :zoom 16.0

-- Remap Enter to clone the last frame rather than inserting a blank one
map :f/clone

-- Set Cavernas palette
p/clear
p/add #3e3d4c
p/add #5a5a63
p/add #847f7f
p/add #bcb19f
p/add #21263f
p/add #394071
p/add #4063b2
p/add #efe7ce
p/add #3a5339
p/add #6b7b39
p/add #84ad42
p/add #c0461f
p/add #6c4f40
p/add #a5845a
p/add #cead73
p/add #673347
```

1. Running `rx` works as expected
2. Running `rx player.png` obeys every command in `.rxrc` except the `zoom 8.0` (whether or not `player.png` exists).

(Which suggests to me that specifying a file on the command line is causing it do something equivalent to injecting a `zoom 1.0` after evaluating `.rxrc`.)