Fix crash when repeatedly opening/closing GUI

Cairo keeps a global cache list of connections, but since the reference
count of the device never reaches 0, the devices are never removed from
this cache list.

The crash happens when a new xcb_connection is made, and the new pointer
happens to be the same as an existing pointer in this cache list, then
the wrong cairo_xcb_connection is returned. Then no screen can be found,
since the root is not the same for that old xcb_connection and the new
one. Thus we end up at ASSERT_NOT_REACHED in _get_screen_index(), in
cairo-xcb-screen.c

Note that there is some resource leakage somewhere, since the reference
count of the cairo devices should reach 0 at some point, but never do.

Fixes steinbergmedia#334

vstgui/lib/platform/linux/cairographicscontext.cpp

@@ -43,6 +43,20 @@ inline cairo_matrix_t convert (const TransformMatrix& ct)
 struct CairoGraphicsDeviceFactory::Impl
 {
 	std::vector<std::shared_ptr<CairoGraphicsDevice>> devices;
+	Impl() = default;
+	~Impl() noexcept
+	{
+		for (auto& device : devices)
+		{
+#if DEBUG
+			DebugPrint ("Cairo device reference count: %u\n", cairo_device_get_reference_count (device->get ()));
+#endif
+			// Force cairo to remove this device from its internal cache list of devices.
+			// This is only needed because the reference count never seem to reach 0,
+			// which means there is still some resource leakage elsewhere in the code.
+			cairo_device_finish (device->get ());
+		}
+	}
 };
 
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