mipmap: Avoid using the vertex backend when GopherJS is not used

The backend logic is especially for GopherJS. As the backend logic
tends to consume a lot of memory, let's avoid this when possible.

Updates #797

image_test.go

@@ -68,10 +68,6 @@ func sameColors(c1, c2 color.RGBA, delta int) bool {
 		abs(int(c1.A)-int(c2.A)) <= delta
 }
 
-func isGopherJS() bool {
-	return web.IsBrowser() && runtime.GOOS != "js"
-}
-
 func TestImagePixels(t *testing.T) {
 	img0, img, err := openEbitenImage()
 	if err != nil {
@@ -549,7 +545,7 @@ func TestImageClear(t *testing.T) {
 func TestImageEdge(t *testing.T) {
 	// TODO: This test is not so meaningful after #1218. Do we remove this?
 
-	if isGopherJS() {
+	if web.IsGopherJS() {
 		t.Skip("too slow on GopherJS")
 		return
 	}
@@ -903,7 +899,7 @@ func TestImageCopy(t *testing.T) {
 
 // Issue #611, #907
 func TestImageStretch(t *testing.T) {
-	if isGopherJS() {
+	if web.IsGopherJS() {
 		t.Skip("too slow on GopherJS")
 		return
 	}

internal/mipmap/vertex.go

@@ -15,9 +15,8 @@
 package mipmap
 
 import (
-	"sync"
-
 	"github.com/hajimehoshi/ebiten/internal/graphics"
+	"github.com/hajimehoshi/ebiten/internal/web"
 )
 
 var (
@@ -29,11 +28,10 @@ var (
 type verticesBackend struct {
 	backend []float32
 	head    int
-	m       sync.Mutex
 }
 
 func (v *verticesBackend) slice(n int, last bool) []float32 {
-	v.m.Lock()
+	// As this is called only from GopherJS, mutex is not required.
 
 	need := n * graphics.VertexFloatNum
 	if l := len(v.backend); v.head+need > l {
@@ -51,14 +49,16 @@ func (v *verticesBackend) slice(n int, last bool) []float32 {
 	} else {
 		v.head += need
 	}
-
-	v.m.Unlock()
 	return s
 }
 
 func vertexSlice(n int, last bool) []float32 {
+	if web.IsGopherJS() {
+		// In GopherJS, allocating memory by make is expensive. Use the backend instead.
 		return theVerticesBackend.slice(n, last)
 	}
+	return make([]float32, n*graphics.VertexFloatNum)
+}
 
 func quadVertices(sx0, sy0, sx1, sy1 int, a, b, c, d, tx, ty float32, cr, cg, cb, ca float32, last bool) []float32 {
 	x := float32(sx1 - sx0)

internal/web/js.go

@@ -17,6 +17,7 @@
 package web
 
 import (
+	"runtime"
 	"strings"
 	"syscall/js"
 )
@@ -25,6 +26,10 @@ func IsBrowser() bool {
 	return true
 }
 
+func IsGopherJS() bool {
+	return IsBrowser() && runtime.GOOS != "js"
+}
+
 var (
 	userAgent = js.Global().Get("navigator").Get("userAgent").String()
 

internal/web/notjs.go

@@ -20,6 +20,10 @@ func IsBrowser() bool {
 	return false
 }
 
+func IsGopherJS() bool {
+	return false
+}
+
 func IsIOSSafari() bool {
 	return false
 }