A few things:
- The library wasn't located in the provided path
- The function pointers weren't using the convention call
- The libraries built from Visual Studio weren't taken into
  account (probably only those generated from the CMake
  generated project were loadable)

Change-Id: I7f9cc9aee920747b4bab77286ba811b33982cece
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33128Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>