Fix Mac Plugin re/sizing

Some plugin UIs, particularly those that cannot be resized
unset `autoresizesSubviews`. Since the plugin-view is re-parented,
the host has to directly set the view's size.

However some plugins have multiple child views, apparently
for off-screen pixmaps (e.g. UAD plugins). Those additional
should not be resized (or re-stacked).

e.g.

Reason-Rack
  view 0x7fe27e44e570 @ 0, 33 834 x 804
    view 0x7fe288aa3770 @ 278, 268 278 x 268

UAD Tube-tech
  view 0x7fe270a9cbf0 @ 0, 33 1160 x 374
    view 0x7fe28883d030 @ 0, 0 1160 x 374
    view 0x7fe2888546e0 @ 0, 0 1160 x 24


see also aef366c156
https://discourse.ardour.org/t/uad-plugin-will-not-load-shows-as-expired-when-its-not/105756/5?u=x42

gtk2_ardour/au_pluginui.mm

@@ -867,6 +867,7 @@ AUPluginUI::cocoa_view_resized ()
 		for (unsigned long i = 0; i < [subviews count]; ++i) {
 			NSView* subview = [subviews objectAtIndex:i];
 			[subview setFrame:NSMakeRect (0, 0, new_frame.size.width, new_frame.size.height)];
+			break; /* only resize first subview */
 		}
 
 		/* also be sure to redraw the topbox because this can
@@ -1092,6 +1093,7 @@ AUPluginUI::parent_cocoa_window ()
 	for (unsigned long i = 0; i < [subviews count]; ++i) {
 		NSView* subview = [subviews objectAtIndex:i];
 		[subview setFrame:NSMakeRect (0, 0, req_width, req_height)];
+		break; /* only resize first subview */
 	}
 
 	last_au_frame = [au_view frame];

gtk2_ardour/mac_vst_plugin_ui.mm

@@ -222,6 +222,7 @@ MacVSTPluginUI::lower_box_size_allocate (Gtk::Allocation& allocation)
 	for (unsigned long i = 0; i < [subviews count]; ++i) {
 		NSView* subview = [subviews objectAtIndex:i];
 		[subview setFrame:NSMakeRect (0, 0, allocation.get_width (), allocation.get_height ())];
+		break; /* only resize first subview */
 	}
 }
 

gtk2_ardour/vst3_nsview_plugin_ui.mm

@@ -153,6 +153,7 @@ VST3NSViewPluginUI::view_size_allocate (Gtk::Allocation& allocation)
 	for (unsigned long i = 0; i < [subviews count]; ++i) {
 		NSView* subview = [subviews objectAtIndex:i];
 		[subview setFrame:NSMakeRect (0, 0, allocation.get_width (), allocation.get_height ())];
+		break; /* only resize first subview */
 	}
 }