ebiten: make some MouseButton definitions explicit

This change redefines named mouse button consts like MouseButtonLeft
to use MouseButton0, so that users can know which enums are the same
and different.

Updates #2445

internal/ui/input_glfw.go

@@ -149,9 +149,9 @@ func (i *Input) Wheel() (xoff, yoff float64) {
 }
 
 var glfwMouseButtonToMouseButton = map[glfw.MouseButton]MouseButton{
-	glfw.MouseButtonLeft:   MouseButtonLeft,
-	glfw.MouseButtonRight:  MouseButtonRight,
-	glfw.MouseButtonMiddle: MouseButtonMiddle,
+	glfw.MouseButtonLeft:   MouseButton0,
+	glfw.MouseButtonRight:  MouseButton1,
+	glfw.MouseButtonMiddle: MouseButton2,
 	glfw.MouseButton3:      MouseButton3,
 	glfw.MouseButton4:      MouseButton4,
 }

internal/ui/input_js.go

@@ -126,9 +126,9 @@ func (i *Input) IsKeyPressed(key Key) bool {
 }
 
 var codeToMouseButton = map[int]MouseButton{
-	0: MouseButtonLeft,
-	1: MouseButtonMiddle,
-	2: MouseButtonRight,
+	0: MouseButton0, // Left
+	1: MouseButton2, // Middle
+	2: MouseButton1, // Right
 	3: MouseButton3,
 	4: MouseButton4,
 }

internal/ui/ui.go

@@ -24,15 +24,11 @@ import (
 type MouseButton int
 
 const (
-	MouseButton0 MouseButton = iota
-	MouseButton1
-	MouseButton2
-	MouseButton3
-	MouseButton4
-
-	MouseButtonLeft   = MouseButton0
-	MouseButtonRight  = MouseButton1
-	MouseButtonMiddle = MouseButton2
+	MouseButton0 MouseButton = iota // The 'left' button
+	MouseButton1                    // The 'right' button
+	MouseButton2                    // The 'middle' button
+	MouseButton3                    // The additional button (usually browser-back)
+	MouseButton4                    // The additional button (usually browser-forward)
 )
 
 type TouchID int

mousebuttons.go

@@ -23,9 +23,10 @@ type MouseButton = ui.MouseButton
 
 // MouseButtons
 const (
-	MouseButtonLeft   MouseButton = ui.MouseButtonLeft
-	MouseButtonRight  MouseButton = ui.MouseButtonRight
-	MouseButtonMiddle MouseButton = ui.MouseButtonMiddle
+	MouseButtonLeft   MouseButton = MouseButton0
+	MouseButtonRight  MouseButton = MouseButton1
+	MouseButtonMiddle MouseButton = MouseButton2
+
 	MouseButton0   MouseButton = ui.MouseButton0
 	MouseButton1   MouseButton = ui.MouseButton1
 	MouseButton2   MouseButton = ui.MouseButton2