Configure ratatui-image preview background/frame color.

[benjajaja]

benjajaja/ratatui-image#21

Due to ratatui's immediate-mode nature, the cells behind the image must be "skipped" so that nothing draws over the image. In other words, the cells behind the image still have whatever was drawn last time before they were marked skipped for the image. Thus, the image must always fill the cells/area, and the frame cannot be made transparent.

The background color should be configurable.