Memory barriers, immutable texture storage, bindless textures, fixes for multisample FBOs, code cleanup, etc.
Oops, I checked in too much
Turn off gl-immutable-texture-storage by default (it seems to cause trouble on NVIDIA cards, which demand a sized internal format be used)
Add atomic adjust impl that uses GCC intrinsics.  Also, make use of "constexpr" when available.
Let's not name members "_object" since it confuses interrogate when parsing initialiser lists.