spin off remaining integer overflow fixes

  - Carmack rsqrt uses an int where it wants a uint32_t.
  - turn off all santizers (including signed-integer-overflow) in third_party/externals/sftntly.

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN-Trybot

BUG=skia:4635

Review URL: https://codereview.chromium.org/1511643002
2 files changed