Fix #3417 and improve handling of `default.tasproj` (squashed PR #3462)

* Fix #3417 and improve handling of default.tasproj

* expose SetMovieController in the MovieSession interface (please don't rely on it anywhere else)

* don't use this explicit public in the interface

(is this mentioned anywhere? i assume this is proper style)

* use this helper function

src/BizHawk.Client.Common/movie/MovieSession.cs

@@ -62,6 +62,11 @@ namespace BizHawk.Client.Common
 			return new Bk2Controller("", definition ?? MovieController.Definition);
 		}
 
+		public void SetMovieController(ControllerDefinition definition)
+		{
+			MovieController = GenerateMovieController(definition);
+		}
+
 		public void HandleFrameBefore()
 		{
 			if (Movie.NotActive())

src/BizHawk.Client.Common/movie/interfaces/IMovieSession.cs

@@ -54,6 +54,17 @@ namespace BizHawk.Client.Common
 		/// </summary>
 		IMovieController GenerateMovieController(ControllerDefinition definition = null);
 
+		/// <summary>
+		/// Hack only used for TAStudio when starting a new movie
+		/// This is due to needing to save a "dummy" default.tasproj
+		/// This dummy file's initial save bypasses the normal queue/run
+		/// new movie code (which normally sets the controller), although
+		/// once it saves it goes through the normal queue/run code anyway.
+		/// TODO: Stop relying on this dummy file so we do not need this ugly hack
+		/// </summary>
+		/// <param name="definition">current IEmulator ControllerDefinition</param>
+		void SetMovieController(ControllerDefinition definition);
+
 		void HandleFrameBefore();
 		void HandleFrameAfter();
 		void HandleSaveState(TextWriter writer);

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.cs

@@ -602,11 +602,16 @@ namespace BizHawk.Client.EmuHawk
 				Config.DefaultAuthor);
 
 			SetTasMovieCallbacks(tasMovie);
+			MovieSession.SetMovieController(Emulator.ControllerDefinition); // hack, see interface comment
 			tasMovie.ClearChanges(); // Don't ask to save changes here.
 			tasMovie.Save();
 			if (HandleMovieLoadStuff(tasMovie))
 			{
 			}
+			// let's not keep this longer than we actually need
+			// the user will be prompted to enter a proper name
+			// when they want to save
+			File.Delete(tasMovie.Filename);
 
 			// clear all selections
 			TasView.DeselectAll();
@@ -780,11 +785,16 @@ namespace BizHawk.Client.EmuHawk
 					filename = SuggestedTasProjName();
 				}
 
-				var file = SaveFileDialog(
-					currentFile: filename,
-					path: Config!.PathEntries.MovieAbsolutePath(),
-					TAStudioProjectsFSFilterSet,
-					this);
+				FileInfo file;
+				do
+				{
+					file = SaveFileDialog(
+						currentFile: filename,
+						path: Config!.PathEntries.MovieAbsolutePath(),
+						TAStudioProjectsFSFilterSet,
+						this);
+				}
+				while (file?.FullName == DefaultTasProjName()); // disallow saving as this reserved filename
 
 				if (file != null)
 				{