Fixes#3084.
This does two things:
1. Cleans up a bunch of places where we used to use `(int)` typecasts
for pointer math, which was problematic. We now use `(INT_PTR)` per
MSDN https://docs.microsoft.com/en-us/windows/win32/winprog64/rules-for-using-pointers
and then cast that down to `(int)` where necessary, e.g. when storing
string lengths which are never going to be more than a few hundred
characters! Doing this explicitly helps to clarify that we are aware
of the typecast and believe it to be safe.
2. Adds in some build infrastructure for future use of Coverity Scan
https://scan.coverity.com/ which we plan to use for further code
quality updates. I have submitted the project to Coverity and are
now waiting for approval so we can check results. Once we have
approval, I do plan to add this to the nightly build (we need to
keep submissions under 3 builds/day).
Note: I have not yet added Keyman Core (Windows) to this project,
nor are we currently building Keyman Core (macOS) or Keyman for
Linux, but we should consider adding those in future.
