io/pointer,gesture: report right and middle mouse button events

Updates gio#60

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/pointer/pointer.go

@@ -5,6 +5,7 @@ package pointer
 import (
 	"encoding/binary"
 	"image"
+	"strings"
 	"time"
 
 	"gioui.org/f32"
@@ -27,6 +28,8 @@ type Event struct {
 	// Time is when the event was received. The
 	// timestamp is relative to an undefined base.
 	Time time.Duration
+	// Buttons are the set of pressed mouse buttons for this event.
+	Buttons Buttons
 	// Hit is set when the event was within the registered
 	// area for the handler. Hit can be false when a pointer
 	// was pressed within the hit area, and then dragged
@@ -86,6 +89,9 @@ type Priority uint8
 // Source of an Event.
 type Source uint8
 
+// Buttons is a set of mouse buttons
+type Buttons uint8
+
 // Must match app/internal/input.areaKind
 type areaKind uint8
 
@@ -116,6 +122,12 @@ const (
 	Grabbed
 )
 
+const (
+	ButtonLeft Buttons = 1 << iota
+	ButtonRight
+	ButtonMiddle
+)
+
 const (
 	areaRect areaKind = iota
 	areaEllipse
@@ -199,4 +211,24 @@ func (s Source) String() string {
 	}
 }
 
+// Contain reports whether the set b contains
+// all of the buttons.
+func (b Buttons) Contain(buttons Buttons) bool {
+	return b&buttons == buttons
+}
+
+func (b Buttons) String() string {
+	var strs []string
+	if b.Contain(ButtonLeft) {
+		strs = append(strs, "ButtonLeft")
+	}
+	if b.Contain(ButtonRight) {
+		strs = append(strs, "ButtonRight")
+	}
+	if b.Contain(ButtonMiddle) {
+		strs = append(strs, "ButtonMiddle")
+	}
+	return strings.Join(strs, "|")
+}
+
 func (Event) ImplementsEvent() {}