Inline and optimize vtkMatrix4x4::Multiply4x4.

Replace the final loop with a call to std::copy, which turns
into memmove at the end of the day.

Change-Id: I3529a949863d4192e83f58d5c86d0df0d8dd1ed1
Merge topic 'matrix-performance-tweak' into master

833e91e8 Inline and optimize vtkMatrix4x4::Multiply4x4.