Improve tutorial program to show how to catch and re-raise a panic.

Some other clarifications added as well.

Co-authored-by: Garrett D'Amore <garrett@damore.org>

TUTORIAL.md

@@ -81,6 +81,10 @@ Button2    | ButtonSecondary | Right button
 Button3    | ButtonMiddle    | Middle button
 Button4    |                 | Side button (thumb/next)
 Button5    |                 | Side button (thumb/prev)
+WheelUp    |                 | Scroll wheel up
+WheelDown  |                 | Scroll wheel down
+WheelLeft  |                 | Horizontal wheel left
+WheelRight |                 | Horizontal wheel right
 
 ## Usage
 
@@ -167,7 +171,6 @@ package main
 import (
 	"fmt"
 	"log"
-	"os"
 
 	"github.com/gdamore/tcell/v2"
 )
@@ -245,12 +248,29 @@ func main() {
 	drawBox(s, 1, 1, 42, 7, boxStyle, "Click and drag to draw a box")
 	drawBox(s, 5, 9, 32, 14, boxStyle, "Press C to reset")
 
+	quit := func() {
+		// You have to catch panics in a defer, clean up, and
+		// re-raise them - otherwise your application can
+		// die without leaving any diagnostic trace.
+		maybePanic := recover()
+		s.Fini()
+		if maybePanic != nil {
+			panic(maybePanic)
+		}
+	}
+	defer quit()
+
+	// Here's how to get the screen size when you need it.
+	// xmax, ymax := s.Size()
+
+	// Here's an example of how to inject a keystroke where it will
+	// be picked up by the next PollEvent call.  Note that the
+	// queue is LIFO, it has a limited length, and PostEvent() can
+	// return an error.
+	// s.PostEvent(tcell.NewEventKey(tcell.KeyRune, rune('a'), 0))
+
 	// Event loop
 	ox, oy := -1, -1
-	quit := func() {
-		s.Fini()
-		os.Exit(0)
-	}
 	for {
 		// Update screen
 		s.Show()
@@ -264,7 +284,7 @@ func main() {
 			s.Sync()
 		case *tcell.EventKey:
 			if ev.Key() == tcell.KeyEscape || ev.Key() == tcell.KeyCtrlC {
-				quit()
+				return
 			} else if ev.Key() == tcell.KeyCtrlL {
 				s.Sync()
 			} else if ev.Rune() == 'C' || ev.Rune() == 'c' {
@@ -272,14 +292,13 @@ func main() {
 			}
 		case *tcell.EventMouse:
 			x, y := ev.Position()
-			button := ev.Buttons()
-			// Only process button events, not wheel events
-			button &= tcell.ButtonMask(0xff)
 
-			if button != tcell.ButtonNone && ox < 0 {
-				ox, oy = x, y
-			}
 			switch ev.Buttons() {
+			case tcell.Button1, tcell.Button2:
+				if ox < 0 {
+					ox, oy = x, y // record location when click started
+				}
+
 			case tcell.ButtonNone:
 				if ox >= 0 {
 					label := fmt.Sprintf("%d,%d to %d,%d", ox, oy, x, y)
@@ -291,3 +310,4 @@ func main() {
 	}
 }
 ```
+