screen: extend interface with Tty method

Extend the Screen interface with a new Tty method, which returns the
underlying tty when the screen is a terminal. This enables direct
drawing to the Tty.

Implement the interface for the three screen implementatinos included in
the library.

screen.go

@@ -259,6 +259,11 @@ type Screen interface {
 	// LockRegion sets or unsets a lock on a region of cells. A lock on a
 	// cell prevents the cell from being redrawn.
 	LockRegion(x, y, width, height int, lock bool)
+
+	// Tty returns the underlying Tty. If the screen is not a terminal, the
+	// returned bool will be false
+	Tty() (Tty, bool)
+
 }
 
 // NewScreen returns a default Screen suitable for the user's terminal

simulation.go

@@ -568,3 +568,7 @@ func (s *simscreen) LockRegion(x, y, width, height int, lock bool) {
 		}
 	}
 }
+
+func (s *simscreen) Tty() (Tty, bool) {
+	return nil, false
+}

tscreen.go

@@ -1868,6 +1868,10 @@ func (t *tScreen) LockRegion(x, y, width, height int, lock bool) {
 	}
 }
 
+func (t *tScreen) Tty() (Tty, bool) {
+	return t.tty, true
+}
+
 // engage is used to place the terminal in raw mode and establish screen size, etc.
 // Think of this is as tcell "engaging" the clutch, as it's going to be driving the
 // terminal interface.

wscreen.go

@@ -577,6 +577,10 @@ func (t *wScreen) LockRegion(x, y, width, height int, lock bool) {
 	}
 }
 
+func (t *wScreen) Tty() (Tty, bool) {
+	return nil, false
+}
+
 // WebKeyNames maps string names reported from HTML
 // (KeyboardEvent.key) to tcell accepted keys.
 var WebKeyNames = map[string]Key{