vkgears: only animate when drawing
While resizing the window, we can end up not rendering due to recreating the swap-chain. If we do all the animation logic right after acquiring the next swap-image, we also reduce the chance of uneven animation pacing.
This is inspired by a patch in !178 (closed), but the author has stopped responding.
Based on top of !200 (merged), because this would otherwise conflict with changes in that MR.