win32: allow multiple windows at the same time

Window classes are process-wide (or at least DLL-wide), so you can't
have 2 classes with the same name. Our code attempted to do this when
for example 2 libmpv instances were created within the same process.
This failed, because RegisterWindowEx() fails if the class already
exists.

Fix this by ignoring RegisterWindowEx() errors. If the class can really
not be registered, we will fail on CreateWindowEx() instead. Of course
we also can't unregister the class, as another thread might be using it.
Windows will free the class automatically if the DLL is unloaded or the
process terminates.

Fixes #2319 (hopefully).