improve border-image: stretch only middle part of images

I have custom images for buttons, which I plug via border-image CSS attribute. they have 1px border and fade-effects near all edges. When the button is large enough, those borders become 2px, and when the button is very small, borders are removed after scaling. also, fade-effects become too stretched (especially if scaling is not proportional).

it would be awesome if Qt stretched only middle part of image (actually, middle-center, middle-bottom, middle-top for horizontal scaling, and middle-center, middle-left, middle-right for vertical scaling).