dri: Refuse to load out-of-tree DRI drivers.

Thanks to glvnd, if we choose to only support libEGL/GL/gbm loading Mesa drivers from the same build tree, then we can stop writing these "public" APIs in dri_interface for the two sides to talk to each other, and instead just use Mesa' internal structures and APIs. Add a version check at driver load time, and start trying to collapse some layers. This MR is mostly a demo of what we can do now, there's a whole lot more cleanup and refactor available once we do I think.

This is a follow-up to !20044 (merged), everything up to glx: Require __DRI_SWRAST >= 4 for doing swrast. should be reviewed over there.