mesa: make compressed-format emulation optional

There's good reasons to emulate compressed formats; mobile and desktop HW generally have different ideas of what formats are worth the extra silicon, so they support different formats based on the legacy they were designed to support. This means we can get better application compatibility by emulating these formats.

But, that comes at a price, namely that applications that supports multiple formats don't really have a way to detect which ones are natively supported and which ones are not. This means they might make the wrong choice, and end up wasting memory and bandwidth when that could have been avoided.

So let's make the emulation optional, by introducing a DRIconf variable to disable it.

This intentionally still leaves in place transcoding support, because that's still probably a win. Besides, those needs to be opted into anyway.