Change teardrop generation to rely more heavily on BOARD_CONNECTIVITY
for improved performance.
Add updating of teardrops on BOARD_COMMIT::Push().
Also converts m_CopperItemRTreeCache to std::shared_ptr.
We don't copy it around anyway, and having to create a new set
of std::unique_ptr's for each operation is likely to be more
expensive than std::shared_ptr's overhead.
This option removes copper layers from pads and vias where they are not
connected to other board elements. This allows the inner layers to be
more closely routed if the via landing pad is not needed.
Fixes https://gitlab.com/kicad/code/kicad/issues/1835
Jeff Young
Wayne Stambaugh
Seth Hillbrand