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/thirdparty/libtsm

@@ -1 +1 @@
-Subproject commit 1f3ae2bc36236136b8ed328b0c64ed954beac33f
+Subproject commit 1fd9cef6f2ddef2609d587ea3a5a8eeadb22a2e4