SwiftShader had over 270 warnings and this
reduces the count to about 90, some of which
can be addressed later, since some are less
trivial to remove. The changes include:
- Making sure we don't build any project with
  RTTI enabled, otherwise, we get some
  vftable size mismatch warnings
- Making sure exception handling is the same
  for all configurations
- Changing minus <unsigned> to the equivalent,
  and valid, ~<unsigned> + 1
- Explicitly casting types to bool by adding
  "!= 0" or "!= nullptr", depending on the case
- Making sure no #define overrules another one
  without first doing the proper #undef of that
  #define
- Removing signed VS unsigned comparisons
- Making sure WinMain is __stdcall

WebGL tests passed.

Change-Id: I8d8d80adc862633b656d7e99c8ea461924fc0ef4
Reviewed-on: https://swiftshader-review.googlesource.com/3475Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>