Support RGBA in background color

[ovaltzer]

Hi there, I couldn't find it in the code and i was wondering if it is possible to add support for

backgroundC := [4]C.double{
		C.double(background.R),
		C.double(background.G),
		C.double(background.B),
		C.double(background.A),
	}

We would like to paint the background of an image with a fully transparent color in case of resizing. Not sure if vips support it, though I was wondering if it's complicated to add? I saw that vips might support: https://stackoverflow.com/questions/32337500/how-to-convert-image-to-rgba-in-vips-tool-how-to-make-sure-two-images-have-th/32349392

Thanks

[h2non]

Happy to help merge a PR implementing this!

[sonal1999]

@ovaltzer Hey have you got the solution for this issue...or any code change that can support transparent background to image.