turnip: Fix support for unaligned render areas.

The previous paths didn't work, they just rendered incorrectly instead of hanging. Instead, load the current contents of the stored attachments so that our aligned store can avoid changing the contents that aren't in the render area.

We could be smarter about having an inner region that doesn't do the loads, but let's start with correct and leave perf_debug()s for when someone cares about the speed of the path that vulkan tells you is a slowpath.

Fixes: ##6327 (closed)