Internally pack buffers are CPU pointers, with an associated
staging texture. In the map call we copy the data out of the
staging memory to CPU memory, and return a pointer to the CPU
buffer.

BUG=angle:511

Change-Id: I2a82addd2f21241d8d49554b2fc71d890a166386
Reviewed-on: https://chromium-review.googlesource.com/195366Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>