Yay, let's do state trackers for every program, so all of my desktop relies on the latest version of Mesa/Gallium, the Kernel and libdrm.
Seriously, it's just flash, not rocket science. Just simply use one of the existing backends for gpu-offloading, try to get the flash-API right and it'll work. I have nearly no CPU/GPU-workload while playing 1080p-Videos on Windows, just get to that state instead of trying insane perfomance enhancement schemes.
Also I bet having Mesa-invasive changes as a dependency will be hell to package, but seeing how Lightspark doesn't even have the goal of fully supporting the Flash-spec(that would be Gnash, which actually works okay, thank you very much fsf), but rather "modern features" of the language, aka whatever the devs feel like that won't matter that much. Gnash ftw.