winmidi: observer returns only USB group terminal blocks

[atsushieno]

I extracted a separate winmidi2 issue from #194:

I am still unable to connect to any of virtual devices using libremidi. It seems like enumerating the virtual devices through libremidi::observer doesn't work. For example, in libremidi examples, the following does not work on my Windows environment:

• run midi2_virtual or run winmidi2 app-to-app sample (mentioned in the earlier comments)
• run midi2_in_winmidi or midi2_out_winmidi -> fails to list the virtual device ^

It is most likely due to the way how winmidi observer finds the ports:

libremidi/include/libremidi/backends/winmidi/observer.hpp

Line 130 in 3f93260

for (const auto& gp : ep.GetGroupTerminalBlocks())

According to winmidi documentation, MidiEndpointDeviceInformation::GetGroupTerminalBlocks() "Returns the currently stored group terminal blocks (USB devices only)".

If I understand correctly, GetDeclaredFunctionBlocks() should be used instead.

[jcelerier]

thanks for finding the bug - and the solution ! I'll be doing windows work soon and implement this