win: return HTCLIENT from WM_NCHITTEST to prevent modal resize freeze

[Shipi1]

• When hosted as a VST3 child window in DAWs (Ableton, etc.), moving the mouse to the bottom/right window border while clicking causes Windows to enter a modal resize loop that freezes the entire plugin window.
• Cause: When MouseDown is active and move the mouse outside the window's border, WM_NCHITTEST is triggered returning HTBOTTOM/HTRIGHT/HTBOTTOMRIGHT
• Fix: wnd_proc_inner always return HTCLIENT since plugin windows are embedded and should never be user-resizable via border dragging. this tells Windows the mouse is always in the client area.
• Tested in Ableton Live 12.3.1

[micahrj]

I'm very surprised that this would be necessary. I would think that the WM_NCHITTEST message would only ever be sent to the top-level window and would not be sent to the plugin's child window in the first place. I also can't find any similar logic in the JUCE or Visage Win32 windowing code. I could be wrong, but I'm somewhat skeptical that this is the correct diagnosis or fix for the problem you are running into.