Reland: graphics: mipmap images should be volatile when the base image is volatile

This is not a bug fix, but should improve performance.

image.go

@@ -284,7 +284,12 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) error {
 		if w2 == 0 || h2 == 0 {
 			break
 		}
-		s := shareable.NewImage(w2, h2)
+		var s *shareable.Image
+		if img.shareableImages[0].IsVolatile() {
+			s = shareable.NewVolatileImage(w2, h2)
+		} else {
+			s = shareable.NewImage(w2, h2)
+		}
 		vs := src.QuadVertices(0, 0, w, h, 0.5, 0, 0, 0.5, 0, 0, nil)
 		is := graphicsutil.QuadIndices()
 		s.DrawImage(src, vs, is, opengl.CompositeModeCopy, graphics.FilterLinear)

internal/restorable/image.go

@@ -99,6 +99,10 @@ func NewScreenFramebufferImage(width, height int) *Image {
 	return i
 }
 
+func (i *Image) IsVolatile() bool {
+	return i.volatile
+}
+
 // BasePixelsForTesting returns the image's basePixels for testing.
 func (i *Image) BasePixelsForTesting() []byte {
 	return i.basePixels

internal/shareable/shareable.go

@@ -323,6 +323,16 @@ func (i *Image) dispose(markDisposed bool) {
 	theBackends = append(theBackends[:index], theBackends[index+1:]...)
 }
 
+func (i *Image) IsVolatile() bool {
+	backendsM.Lock()
+	defer backendsM.Unlock()
+	if i.backend == nil {
+		// Not allocated yet. Only non-volatile images can do lazy allocation so far.
+		return false
+	}
+	return i.backend.restorable.IsVolatile()
+}
+
 func (i *Image) IsInvalidated() (bool, error) {
 	backendsM.Lock()
 	defer backendsM.Unlock()