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.

examples/asciicast/asciicast.tscn

@@ -1,20 +1,16 @@
-[gd_scene load_steps=4 format=2]
+[gd_scene load_steps=3 format=2]
 
-[ext_resource path="res://addons/godot_xterm/themes/default_dark.tres" type="Theme" id=1]
-[ext_resource path="res://addons/godot_xterm/nodes/terminal/terminal.gdns" type="Script" id=4]
+[ext_resource path="res://addons/godot_xterm/nodes/terminal/terminal.gd" type="Script" id=1]
 [ext_resource path="res://examples/asciicast/example.cast" type="Animation" id=6]
 
 [node name="Terminal" type="Control"]
 anchor_right = 1.0
 anchor_bottom = 1.0
-focus_mode = 2
-theme = ExtResource( 1 )
-script = ExtResource( 4 )
+focus_mode = 1
+script = ExtResource( 1 )
 __meta__ = {
 "_edit_use_anchors_": false
 }
-rows = 31
-cols = 102
 
 [node name="AnimationPlayer" type="AnimationPlayer" parent="."]
 autoplay = "example"