qt: use more reasonable source size for artist banner background image

As long as the interface window is a fraction of the screen width, the displayed image is smaller than the source size, making it hard to justify having such big source size. And when the interface window is big, the details are gone due to blurring, which again makes it hard to justify having big source (hence image, hence texture) size.

Granted, this is going to make the background more blurry when the interface window width is bigger than half of the screen size. But, the increase in blurriness in that case is substantially less than what the blur effect already achieves. Though with 1/4 it was rather obvious, so 1/2 should be safer.

This change reduces video and system memory consumption considerably.

Also see !7533, where I proposed using a common image and texture for the background and the round image, further reducing the video and system memory consumption.