Callback for Jimp.rgbaToInt should be optional

[HanKruiger]

I'm getting the following TypeScript error:

even though I should be able to use the method as such (from the README):

Jimp.rgbaToInt(r, g, b, a); // e.g. converts 255, 255, 255, 255 to 0xFFFFFFFF

I think the error occurs because of a missing ? in the typings:

rgbaToInt:
https://github.com/oliver-moran/jimp/blob/585c1aba7ef2967eb948486b047ba766d761d9f0/packages/core/types/jimp.d.ts#L102

I think that's the issue, because for, for example, intToRGBA the callback is optional:
https://github.com/oliver-moran/jimp/blob/585c1aba7ef2967eb948486b047ba766d761d9f0/packages/core/types/jimp.d.ts#L104

Relatively new to TypeScript, so I definitely could be missing something.

Cheers!