Clean up copyvertex.h

 * Move "private" functions into copyvertex.inl.
 * Capitalize global function names give them more descriptive names.
 * Use size_t for all dimensions.
 * Use uint8_t pointers instead of unsigned char pointers.

Change-Id: I408cf7ebb28655c5811ad18979a20f4273d278cd
Reviewed-on: https://chromium-review.googlesource.com/207373
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>