restorable: Add ResetRestoringState to reset the state

After Fill command, the image doesn't have to keep the restoring
information. Now Fill command is as same as DrawTriangles, there
is no way for restorable.Image to know whether it can reset the
state or not. ResetRestoringState clears the state explicitly.

image.go

@@ -127,6 +127,10 @@ 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()
 	i.DrawImage(emptyImage, op)
 	return nil
 }

internal/restorable/image.go

@@ -224,7 +224,14 @@ 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

@@ -312,6 +312,15 @@ 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

@@ -112,6 +112,14 @@ func (m *mipmap) disposeMipmaps() {
 	}
 }
 
+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 returns -1 if det is 0.

uicontext.go

@@ -111,7 +111,7 @@ func (c *uiContext) Update(afterFrameUpdate func()) error {
 	}
 
 	// This clear is needed for fullscreen mode or some mobile platforms (#622).
-	c.screen.mipmap.orig.ClearFramebuffer()
+	c.screen.mipmap.clearFramebuffer()
 
 	op := &DrawImageOptions{}