[Mesa-dev] Apply blorp and userptr to readback

[Chris Wilson]

This series isn't yet fully baked, but I expect you can point out
approaches that need to be reworked already...

The starting point was to avoid the abysmal readback performance on !llc,
but with a simple application of blorp we get a lot of format conversions
for "free". (The only drawback is that blorp isn't a complete GL meta
pipeline so we still have to apply some fallbacks, and ofc no every
possible user format is supported by hw.)
-Chris