Missing validation for mapped buffers and overlap
was added to CopyBufferSubData.

Change-Id: If1f3f9901f039f5737d41ca46d98ce4c188274d6
Reviewed-on: https://swiftshader-review.googlesource.com/3603Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>