loader/dri3: do not blit outside old/new buffers
This was discovered while hacking on Zink, where out-of-range blits cause validation errors.
It seems better to solve this problem at the root cause than fixing it in each driver.
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
This was discovered while hacking on Zink, where out-of-range blits cause validation errors.
It seems better to solve this problem at the root cause than fixing it in each driver.