graphics: Reland: Bug fix: a too old frame was rendered at least on Android

Fixes #525

graphicscontext.go

@@ -16,6 +16,7 @@ package ebiten
 
 import (
 	"github.com/hajimehoshi/ebiten/internal/clock"
+	"github.com/hajimehoshi/ebiten/internal/graphics"
 	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/restorable"
 	"github.com/hajimehoshi/ebiten/internal/ui"
@@ -97,17 +98,22 @@ func (c *graphicsContext) Update(afterFrameUpdate func()) error {
 		}
 		afterFrameUpdate()
 	}
-	if 0 < updateCount {
-		// Call ClearFramebuffer instead of c.screen.Clear()
+
+	// Clear the screen framebuffer by DrawImage instad of Fill
 	// to clear the whole region including fullscreen's padding.
-		c.screen.restorable.ClearFramebuffer()
+	op := &DrawImageOptions{}
+	w, h := emptyImage.Size()
+	// graphics.MaxImageSize should be the maximum size of framebuffer.
+	op.GeoM.Scale(graphics.MaxImageSize/float64(w), graphics.MaxImageSize/float64(h))
+	op.CompositeMode = CompositeModeCopy
+	op.Filter = filterScreen // any filter is fine: just use the same filter as below.
+	c.screen.DrawImage(emptyImage, op)
 
 	dw, dh := c.screen.Size()
 	sw, _ := c.offscreen.Size()
 	scale := float64(dw) / float64(sw)
 
-		op := &DrawImageOptions{}
-
+	op = &DrawImageOptions{}
 	// c.screen is special: its Y axis is down to up,
 	// and the origin point is lower left.
 	op.GeoM.Scale(scale, -scale)
@@ -117,7 +123,6 @@ func (c *graphicsContext) Update(afterFrameUpdate func()) error {
 	op.CompositeMode = CompositeModeCopy
 	op.Filter = filterScreen
 	_ = c.screen.DrawImage(c.offscreen, op)
-	}
 
 	if err := restorable.ResolveStaleImages(); err != nil {
 		return err

internal/restorable/image.go

@@ -129,10 +129,6 @@ func (i *Image) clearIfVolatile() {
 	if !i.volatile {
 		return
 	}
-	i.ClearFramebuffer()
-}
-
-func (i *Image) ClearFramebuffer() {
 	i.basePixels = nil
 	i.drawImageHistory = nil
 	i.stale = false