Update to 0.18.4:

Log:
        S淡ren Sandmann Pedersen (6):
              Post-release version bump to 0.18.3
              Fix memory leak in the pthreads thread local storage code
              When storing a g1 pixel, store the lowest bit, rather than comparing with
              If we bail out of do_composite, make sure to undo any workarounds.
              Check for read accessors before taking the bilinear fast path
              Pre-release version bump to 0.18.4

(wiz)