Improve DLL loading behavior on Windows

[notr1ch]

Description

Improves how we load various DLLs by preferring system32 DLLs, removing global SetDllDirectory changes, enabling safe path search mode and reducing risk of loading potentially wrong DLLs from the current directory.

Motivation and Context

Annoyed by various bug reports that suggest OBS is vulnerable to DLL planting attacks. While this PR doesn't aim to solve that issue directly, it implements best practices to help reduce the risk. We still rely on the current directory for relative path generation which isn't ideal, but that's for another PR.

How Has This Been Tested?

Tested loading OBS with default set of plugins. More testing with 3rd party plugins with complex dependency chains might be needed, though LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR should match the old behavior.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.