WIP: Add tweak interface

This series adds the possibility to receive requests to apply application specific tweaks. One use case will be to fix #91 (closed) by changing the blit behavior, but this will need !173 (merged) to be merged first, and #94 (closed) by emulating a BGRA_SRGB render target.

The mesa side is here: mesa/mesa!542 (closed)

With respect to the BGRA tweaks I'm also thinking of another option where we tell the guest that BGRA_SRGB is supported but only really expose this by using a client side tweak, and then do emulation on the host when needed. This might give us a more fine grained control about when BGRA_sRGB should be done.