qt: miscellaneous adjustments in ArtistTopBanner

• Limit loaded image size, like it is done in Player.qml.
• Enable mipmapping, considering texture atlas is not relevant here. Mip maps will make it scale better, as well as ease sampling for the blur effect.
• Fix the input image getting shown below the effect output.
• Use FastBlur instead of GaussianBlur. I can also use cheap "Box Blur" here, if there is room to sacrifice more quality, please let me know.
• Some layout adjustments, such as limiting text width.
• Disable unnecessary clipping (?) I'm not sure about that, why was this needed in the first place?