fix: icon generator color type error

[CatchABus]

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What is the current behavior?

Right now, cli cannot generate assets the alpha of which is disabled.
Output:

ns resources generate icons ./icon.png

Generating icons ...
image.colorType is not a function

This behaviour broke during 8.9 update and update of Jimp dependency.

What is the new behavior?

Cli will be able to generate assets the alpha of which is disabled (rgba: true).