Provide a way to deal with transparency

[gross-nvs]

Hello,

I encountered an annoying case where I'm pulling images from a source using a black transparent background and haven't found any way to deal with it.

When I try to save the image without alpha after a fit() and a background() the resulting image has a black background instead of the background of my choice, on both drivers (aka not a bug).

Here is an example with a red dot over a transparent black rgba(0, 0, 0, 0) :

Using the following code:

Image::load($image)
        ->orientation()
        ->fit(Fit::Fill, 300, 300)
        ->background('rgba(255, 255, 255, 0)') // Try to enforce a white background when alpha channel is not supported
        ->save('imagick.jpg');

Will result in this:

It will be nice if we could choose how insert() handles the alpha channel or a way to define a background color when saving without alpha channel.