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.

test/scenes/basic.gd

@@ -0,0 +1,10 @@
+extends Control
+
+onready var terminal = $Terminal
+
+
+func _ready():
+	print("terminal size; rows %d; cols %d;" % [terminal.rows, terminal.cols])
+	terminal.write("h")
+	yield(get_tree().create_timer(1), "timeout")
+	terminal.write(" i")

test/scenes/basic.tscn

@@ -0,0 +1,29 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://addons/godot_xterm/nodes/terminal/terminal.gd" type="Script" id=2]
+[ext_resource path="res://test/scenes/basic.gd" type="Script" id=3]
+
+[node name="BasicNewTerm" type="Control"]
+margin_right = 43.0
+margin_bottom = 43.0
+script = ExtResource( 3 )
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="ColorRect" type="ColorRect" parent="."]
+margin_right = 40.0
+margin_bottom = 40.0
+color = Color( 0, 1, 0, 1 )
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="Terminal" type="Control" parent="."]
+anchor_right = 1.0
+anchor_bottom = 1.0
+focus_mode = 1
+script = ExtResource( 2 )
+__meta__ = {
+"_edit_use_anchors_": false
+}

test/scenes/basic_fullscreen.tscn

@@ -0,0 +1,21 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://addons/godot_xterm/nodes/terminal/terminal.gd" type="Script" id=1]
+[ext_resource path="res://test/scenes/basic.gd" type="Script" id=2]
+
+[node name="BasicNewTerm" type="Control"]
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource( 2 )
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="Terminal" type="Control" parent="."]
+anchor_right = 1.0
+anchor_bottom = 1.0
+focus_mode = 1
+script = ExtResource( 1 )
+__meta__ = {
+"_edit_use_anchors_": false
+}