graphics: Remove 'resetRestoringState'

This is basically a revert of 2d079b123f

As filling can happen on a part of a shared image, resetting the
restoring state was wrong.

image.go

@@ -123,10 +123,7 @@ func (i *Image) Fill(clr color.Color) error {
 	if af < 1.0 {
 		op.CompositeMode = CompositeModeCopy
 	}
-	// As Fill will change all the pixels of the image into the same color, all the information for restoring
-	// will be invalidated.
-	// TODO: This is a little hacky. Is there a better way?
-	i.mipmap.resetRestoringState()
+	// TODO: As all the pixels will be changed, this image can reset the information for restoring.
 	i.DrawImage(emptyImage, op)
 	return nil
 }

internal/restorable/image.go

@@ -223,14 +223,7 @@ func (i *Image) clear() {
 	}
 
 	clearImage(i.image)
-	i.ResetRestoringState()
-}
 
-// ResetRestoringState resets all the information for restoring.
-// ResetRestoringState doen't affect the underlying image.
-//
-// After ResetRestoringState, the image is assumed to be cleared.
-func (i *Image) ResetRestoringState() {
 	i.basePixels = Pixels{}
 	i.drawTrianglesHistory = nil
 	i.stale = false

internal/shareable/shareable.go

@@ -342,15 +342,6 @@ func (i *Image) ClearFramebuffer() {
 	i.backend.restorable.Clear()
 }
 
-func (i *Image) ResetRestoringState() {
-	backendsM.Lock()
-	defer backendsM.Unlock()
-	if i.backend == nil {
-		return
-	}
-	i.backend.restorable.ResetRestoringState()
-}
-
 func (i *Image) ReplacePixels(p []byte) {
 	backendsM.Lock()
 	defer backendsM.Unlock()

mipmap.go

@@ -144,10 +144,6 @@ func (m *mipmap) clearFramebuffer() {
 	m.orig.ClearFramebuffer()
 }
 
-func (m *mipmap) resetRestoringState() {
-	m.orig.ResetRestoringState()
-}
-
 // mipmapLevel returns an appropriate mipmap level for the given determinant of a geometry matrix.
 //
 // mipmapLevel panics if det is NaN or 0.