Fix some more clang warnings

[Lgt2x]

Pull Request Type

• [ ] GitHub Workflow changes
• [ ] Documentation or Wiki changes
• [x] Build and Dependency changes
• [ ] Runtime changes
  • [ ] Render changes
  • [ ] Audio changes
  • [ ] Input changes
  • [ ] Network changes
  • [ ] Other changes

Description

• Fix -Wnull-conversion using explicit false
• Fix -Wnon-c-typedef-for-linkage warning using C++ struct declaration syntax
• Fix double extern qualifier warning in idmfc.h. Functions declared in idmfc.h were prefixed by both macros EXTERN and DLLEXPORT. EXTERN expands to extern "C" for C++ code, and DLLEXPORT expands to extern "C" for C++ or extern for C code, making up two extern qualifiers for C++ code. We now only use DLLEXPORT macro.
• Fix compiler warning -Wimplicit-const-int-float-conversion using explicit casts

[winterheart]

Runs fine on Linux.