Ardour seems to listen to the bounds of the plugin window, and will call
`onSize` on the plugin editor when move/resize events are sent to the X
window - even if the size of the window didn't really change. This can
result in an infinite resize loop, where calling `onSize` on the VST3
instance sends a resize event to the plugin window, and this event
causes Ardour to call `onSize` on the plugin view.

To get around this, the Linux ComponentPeer will no longer request a
bounds change from the window system if the requested bounds are the
same as the current bounds.

Previously, things like PopupMenus which were created in their own
windows were not being scaled correctly on HiDPI displays on Linux.

This patch forces the display scale to 1.0 in plugins, meaning that
the transform applied to the main plugin window is the sole source
of truth for component scaling in plugins.

It seems like shared_from_this may not be enabled when a unique_ptr is
assigned to a shared_ptr (although it *should* be enabled when
constructing a new shared_ptr from a unique_ptr). Functions that return
objects that may need to use shared_from_this now return shared_ptr,
just to be safe.

Additionally, in some cases, shared_from_this was being called from
Thread::run after the last reference to the shared object had been
released. We now call shared_from_this during 'open', which will always
run on the message thread while at least once reference to the shared
object is alive.