Fix spurious "?" from keys like backspace on Linux

Input keys such as backspace, home, end, can insert spurious "?" characters on Linux. The fix is simply to stop processing input on this frame once these keys have already been handled.
This is an old bug that returned in 0.16.1 after IME refactor of TextBox control. This change restores the fix method.

Assets/Scripts/Game/UserInterface/TextBox.cs

@@ -211,8 +211,13 @@ public override void Update()
                 return;
             }
 
-            // Process input
-            HandleControlInput();
+            // Process control input
+            // Exit if this input was handled by below call
+            // Otherwise keys like backspace can insert spurious "?" characters on some platforms
+            if (HandleControlInput())
+                return;
+
+            // Process all other input
             HandleCharacterInput();
             UpdateInlineComposition();
 
@@ -340,57 +345,67 @@ string GetResultText()
                 return Text;
         }
 
-        // Handle control input such as for cursor movement, delete/backspace, home/end
-        void HandleControlInput()
+        // Handle control input such as cursor movement, delete/backspace, home/end
+        // Returns true when control input was handled by this method or false when input processing should continue
+        // The caller should stop processing input on this loop when this method returns true
+        // And continue processing input on this loop when this method returns false
+        bool HandleControlInput()
         {
-            // During IME composition these kays are handed over to provider
+            // During IME composition these keys are always handed over to provider
             if (IMECompositionInProgress)
-                return;
+                return false;
 
             // Moving cursor left and right
-            if (DaggerfallUI.Instance.LastKeyCode == KeyCode.LeftArrow)
+            KeyCode lastKeyCode = DaggerfallUI.Instance.LastKeyCode;
+            if (lastKeyCode == KeyCode.LeftArrow)
             {
                 MoveCursorLeft();
-                return;
+                return true;
             }
-            else if (DaggerfallUI.Instance.LastKeyCode == KeyCode.RightArrow)
+            else if (lastKeyCode == KeyCode.RightArrow)
             {
                 MoveCursorRight();
-                return;
+                return true;
             }
 
             // Delete and Backspace
-            if (DaggerfallUI.Instance.LastKeyCode == KeyCode.Delete)
+            if (lastKeyCode == KeyCode.Delete)
             {
                 if (cursorPosition != text.Length)
                 {
                     text = text.Remove(cursorPosition, 1);
                     RaiseOnTypeHandler();
                 }
-                return;
+                return true;
             }
-            else if (DaggerfallUI.Instance.LastKeyCode == KeyCode.Backspace)
+            else if (lastKeyCode == KeyCode.Backspace)
             {
                 if (cursorPosition != 0)
                 {
                     text = text.Remove(cursorPosition - 1, 1);
                     MoveCursorLeft();
                     RaiseOnTypeHandler();
                 }
-                return;
+                return true;
             }
 
             // Home and End
-            if (DaggerfallUI.Instance.LastKeyCode == KeyCode.Home)
+            if (lastKeyCode == KeyCode.Home)
             {
                 SetCursorPosition(0);
-                return;
+                return true;
             }
-            else if (DaggerfallUI.Instance.LastKeyCode == KeyCode.End)
+            else if (lastKeyCode == KeyCode.End)
             {
                 SetCursorPosition(text.Length);
-                return;
+                return true;
             }
+
+            // Escape - just absorb this so escape does not send spurious input into textbox
+            if (lastKeyCode == KeyCode.Escape)
+                return true;
+
+            return false;
         }
 
         // Handling for basic character input or IME composition insert