Improve heightfield collision performance

[glebm]

In openmw, btHeightfieldTerrainShape::processAllTriangles is 15% of the total frame time

I've noticed the following TODO in the code that points to the possibility of optimizing this:

bullet3/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.cpp

Line 336 in abea1a8

// TODO If m_vboundsGrid is available, use it to determine if we really need to process this area

Heightfield accelerator data structure (m_vboundsGrid) was introduced in #2062

There are a few other TODOs in the file that also indicate missing optimizations.

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Update: No longer a bottleneck even without the accelerator thanks to #3287. Leaving open for now in case someone wants to do address the TODOs and claim the bounty.

/cc @Zylann @psi29a

[glebm]

Posted a bounty https://www.bountysource.com/issues/96668355-heighfield-collision-performance

[glebm]

After #2062, in areas where heightfield collision is still slow, perf report looks like this: