Fix unnecessary painting in CoverArtDelegate

[daschuer]

I have created this during my my first attempt to fix the priority inversion #11128 when scrolling with cover arts column enabled. This is a bit of a speed up, but does not fix the actual problem. This will be part of the next PR.

[ronso0]

FWIW I'm not noticing a difference in performance between 2.4 and this.
With both branches, for covers that are skipped while scrolling fast (only the color is painted) the images are loaded only when I scroll back to the skipped rows. Ie. I don't see any effect of c36e566/6c841bd4b2320f6dd4ebf0599c4f3d6697caf9e4 (IIUC how those are supposed to work).

[ronso0]

Just some small findings, I'll take a closer look soonish.

[daschuer]

The actual but small speed up happens once you stop scrolling. In this case the cover pictures appears faster. In the original version the uni-color replacements are first painted again after scroll stop and than the original covers are fetched and pained.

The other optimizations has only a negligible speed up. In 2.4 the chache grows up to 1000 and more, which requires repeated memory allocations. This branch reuses the memory once it is allocated for the number of all visible rows.

[daschuer]

Done

[daschuer]

Done