QCosmeticStroker: fix several UBs involving << with a negative LHS

Left-shifts of negative values are undefined in C++. In particular,
they don't behave arithmetically.

Reported by UBSan:

  qcosmeticstroker.cpp: 72:15: runtime error: left shift of negative value -14/-19/-32/-33/-34/-37/-38/-63/-64/-192/-384/-1280
  qcosmeticstroker.cpp:444:20: runtime error: left shift of negative value -64
  qcosmeticstroker.cpp:451:26: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:483:26: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:762:20: runtime error: left shift of negative value -64
  qcosmeticstroker.cpp:774:26: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:813:47: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:839:20: runtime error: left shift of negative value -64
  qcosmeticstroker.cpp:851:26: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:889:47: runtime error: left shift of negative value -1
  qcosmeticstroker.cpp:932:27: runtime error: left shift of negative value -64
  qcosmeticstroker.cpp:995:27: runtime error: left shift of negative value -3/-64

Fix by using ordinary multiplication instead, because negative
left-hand-side values don't look like they are an error.

Change-Id: Icbebd41f6ddd3dca4abd385585fc0f82064fe8b6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>