Set image source sizes to a fixed value in HeaderBar
Set the background source size to a fixed value as otherwise window resizing will cause the background image to be reloaded causing flicker and slowness. Fix the value to an arbitrarily chosen, rather low size to reduce memory size. The large blur does not need a high resolution image. The cover art is also set to a fixed size to reduce flickering. Mipmapping is enabled to maintain good image quality when the default cover art SVG is shown.