Error: undeclared identifier: 'Rect'

Thanks for this great library!

This error is on Linux (docker nim-lang/nim:1.4.2). The same code seems to work fine on macOS

# cat foo.nim 
import os
import flippy

proc resizePNG*(src:string, dst:string, width:int, height:int) =
  loadImage(src).resize(width, height).save(dst)

when isMainModule:
  resizePNG(paramStr(1), paramStr(2), 100, 100)

# nim c foo.nim 
Hint: used config file '/nim/config/nim.cfg' [Conf]
Hint: used config file '/nim/config/config.nims' [Conf]
........................................
/root/.nimble/pkgs/flippy-0.4.7/flippy.nim(311, 42) Error: undeclared identifier: 'Rect'

# nimble list -i
chroma  [0.1.5]
flippy  [0.4.7]
nimPNG  [0.3.1]
supersnappy  [2.0.0]
vmath  [0.4.0]

# nim --version
Nim Compiler Version 1.4.2 [Linux: amd64]
Compiled at 2020-11-30
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 3fb5157ab1b666a5a5c34efde0f357a82d433d04
active boot switches: -d:release -d:danger

# uname -a
Linux 883414a58802 5.4.39-linuxkit #1 SMP Fri May 8 23:03:06 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

treeform / flippy

Error: undeclared identifier: 'Rect' #32