Fixed functionality of #from_file within non-main Ractors, #170

[TheHashTableSlasher]

See issue #170 for more information. I tried to provide a useful spec to test this, but I'm pretty busy and didn't have time to set up a whole thing on the PC that I'm writing this from, so you may want to move it somewhere else or something.

[wvanbergen]

Nice, thanks for this contribution. I think I prefer the freeze approach - it's a good idea to freeze those constants for other reasons besides Ractors. So I'll merge this PR.

[wvanbergen]

Looks like the test suite is showing some bitrot, but one failure seems to be related to this change:

/home/runner/work/chunky_png/chunky_png/spec/chunky_png/canvas_spec.rb:242:in `block (3 levels) in <top (required)>': undefined method `resource_file' for #<Ractor:#2 /home/runner/work/chunky_png/chunky_png/spec/chunky_png/canvas_spec.rb:242 running> (NoMethodError)

    img = Ractor.new { ChunkyPNG::Canvas.from_file(resource_file("operations.png")) }.take

Looks like the resource_file is not available in the Ractor context - can you look into it?

[TheHashTableSlasher]

Sorry, never used RSpec before. It oughtta work now though.