Use the shared cache system introduced in e8f4010611 for the
"looptris" triangulation cache. This avoids recalculation when meshes
are copied but the positions or topology don't change. The most obvious
improvement is for cases like a large meshes being adjusted slightly
with a simple geometry nodes modifier. In a basic test with a transform
node with a 1 million point grid I observed an improvement of 13%, from
9.75 to 11 FPS, which shows that we avoid spending 6ms recalculating
the triangulation of every update.
This also makes the thread safety for the triangulation data use a
more standard double-checked lock pattern, which is nice because we
can avoid holding a lock whenever the cached data is retrieved.
Split from https://developer.blender.org/D16530
511ac66dab