HACK: winex11: Disable host window decorations with Mutter.

Mutter is utterly broken when changing decorations. It's better not to
have decorations at all and use Windows frames for the very few cases
they would still be visible.

CW-Bug-Id: #24939

Author: rbernon

dlls/winex11.drv/window.c

@@ -472,6 +472,8 @@ static unsigned long get_mwm_decorations_for_style( DWORD style, DWORD ex_style
 {
     unsigned long ret = 0;
 
+    if (X11DRV_HasWindowManager( "Mutter" )) return 0;
+
     if (ex_style & WS_EX_TOOLWINDOW) return 0;
     if (ex_style & WS_EX_LAYERED) return 0;