Similar to unpack state, the PixelPackState struct encapsulates
all the parameters related to pixel pack buffers. Passing the
packstate makes functions more consise.

BUG=angle:511

Change-Id: If6954a5085e9b8f828cfc8892a73e7c7514b0c8a
Reviewed-on: https://chromium-review.googlesource.com/191032Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>