Multiple updates

- Use viewport as render target for terminal:
  Terminal now only draws cells which changed since the last _draw() call.
  A viewport is used with clear mode set to NEVER to cache previous draw
  calls. The terminal node and viewport are wrapped by a GDScript Terminal
  node which takes care of resizing the viewport scene, and forcing the
  terminal to redraw all cells when necessary (i.e. on resize or theme
  change).

  Adds update_mode to terminal interface which can be set to one of:
  - DISABLED: terminal will never be drawn
  - AUTO: terminal will only draw the cells that changed, but
    automatically redraw the full screen when necessary (for example,
    when the size or theme changed).
  - ALL: terminal will always draw every cell on every update. This is
    the most reliable but least performant option.
  - ALL_NEXT_FRAME: Will use update_mode ALL for the next _draw() call,
    then change update_mode back to AUTO.

- Upgraded libtsm:
  Includes changes from Fredrik Wikstrom (salass00)'s fork of libtsm.

- Don't require theme to be set.
  Terminal will use default fonts/colors if no theme is set.

addons/godot_xterm/native/src/terminal.h

@@ -14,18 +14,6 @@ class Terminal : public Control {
   GODOT_CLASS(Terminal, Control)
 
 public:
-  struct cell {
-    char ch[5];
-    struct tsm_screen_attr attr;
-  };
-  static const struct cell empty_cell;
-
-public:
-  typedef std::vector<std::vector<struct cell>> Cells;
-  typedef std::vector<struct cell> Row;
-
-  Cells cells;
-
   Ref<InputEventKey> input_event_key;
 
 protected:
@@ -44,11 +32,13 @@ private:
   std::map<String, Ref<Font>> fontmap = {};
 
   void update_size();
-
   void update_theme();
-  std::pair<Color, Color> get_cell_colors(int row, int col);
+
+public:
+  std::pair<Color, Color> get_cell_colors(const tsm_screen_attr *attr);
   void draw_background(int row, int col, Color bgcol);
-  void draw_foreground(int row, int col, Color fgcol);
+  void draw_foreground(int row, int col, char *ch, const tsm_screen_attr *attr,
+                       Color fgcol);
 
 public:
   static void _register_methods();
@@ -64,10 +54,16 @@ public:
 
   void write(Variant data);
 
+  enum UpdateMode {
+    DISABLED,
+    AUTO,
+    ALL,
+    ALL_NEXT_FRAME,
+  };
+
   int rows;
   int cols;
-
-  bool sleep;
+  int update_mode;
 
   uint8_t color_palette[TSM_COLOR_NUM][3];