[LinearSolver.Direct] BTDLinearSolver: do not clear the dense inverse at each factorization - #6433

invert() resized Minv at every factorization, and BlockFullMatrix::resize() always clears all its nb*nb blocks.

It appeared that clear() seems superfluous, as the block as always written before being read.
Empirical checks:

- Every write is a plain assignment.
- computeMinvBlock only reads blocks it has already written
- getMinvElement → computeMinvBlock before reading. 

and Claude tested by filling the Minv with NaN instead of clearing -> 0 NaN reads.

Not really a real proof but seems OK for me © 😅

Results: use case in BeamAdapter,
Iin 3instruments_collis, the default tools has 180 nodes, results are roughly similar

master: 5000 iterations done in 21.9278 s ( 228.021 FPS)
PR:     5000 iterations done in 21.4084 s ( 233.553 FPS)

if we increase the number of nodes (only in the mechanical object) to e.g 580 nodes

master 5000 iterations done in 34.5094 s ( 144.888 FPS).
PR     5000 iterations done in 26.2973 s ( 190.134 FPS).

and 1080 nodes

master 5000 iterations done in 62.2104 s ( 80.3724 FPS).
PR.    5000 iterations done in 33.0253 s ( 151.399 FPS)

my first version was introducing a Data to force clearing anyway (by default) to keep the compatibility, but removed it anyway as it seems totally useless.

Note that it will clear anyway if the size is different between factorization

[with-all-tests]

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