Allow Background Color overwrite when converting PNG images to JPG.

[francoislevesque]

Is your feature request related to a problem? Please describe.

We need a white background when converting PNG images to JPG.
Currently, using SharpJS, the default fill background is black. I believe this feature would benefit many other users.

Describe the solution you would like

1. It could be a config settings on the Craft Admin panel. Ex: Default fill color: #ffffff.
2. It could also be interesting to send a "sharpjs" parameter when creating an image URL. Ex:

// Everything in the sharpjs key gets merged with the SharpJS config.
image.url({width: 400,format: 'jpg', quality: 75, sharpjs: {flatten: {background: 'rgba(255,255,255)'}}}

Describe alternatives you have considered

I tried sending custom sharpjs parameters to the image URL generator, but found out the package doesn't use/merge custom parameters.

Thanks for the support and the great package 😊