Implementation of buffer map for pack buffers.

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/195366
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
(cherry picked from commit 0b44a455b5a81b84662e57f30348f4e791f62358)
Reviewed-on: https://chromium-review.googlesource.com/197272
2 files changed