Performance tweaks to Glyph3D.

Use batch copies where possible. This reduces the runtime on an
input with ~1M points by 6% (2.41 --> 2.27).

Change-Id: I6bc8de966724064bbb84e857ac35fb74088ef7b4
Merge topic 'glyph3d-batch-copies' into master

36fd8c7c Performance tweaks to Glyph3D.