Don't second guess the visible portion of a FillOriginal

Just skip frame 1 the flicker goes :)

Fixes fyne-io#1432

internal/painter/image.go

@@ -55,26 +55,23 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 				aspect := float32(origW) / float32(origH)
 				viewAspect := float32(width) / float32(height)
 
-				firstPassOriginal := false
+				texW, texH := width, height
+				if viewAspect > aspect {
+					texW = int(float32(height) * aspect)
+				} else if viewAspect < aspect {
+					texH = int(float32(width) / aspect)
+				}
+
+				icon.SetTarget(0, 0, float64(texW), float64(texH))
+				// this is used by our render code, so let's set it to the file aspect
+				aspects[img.Resource] = aspect
 				// if the image specifies it should be original size we need at least that many pixels on screen
 				if img.FillMode == canvas.ImageFillOriginal {
 					if !checkImageMinSize(img, c, origW, origH) {
-						firstPassOriginal = true
+						return nil
 					}
 				}
 
-				texW, texH := width, height
-				if firstPassOriginal { // we draw pixel 1:1 so we can subimage it below to fit in the current bounds
-					texW, texH = origW, origH
-				} else {
-					if viewAspect > aspect {
-						texW = int(float32(height) * aspect)
-					} else if viewAspect < aspect {
-						texH = int(float32(width) / aspect)
-					}
-				}
-
-				icon.SetTarget(0, 0, float64(texW), float64(texH))
 				tex = image.NewNRGBA(image.Rect(0, 0, texW, texH))
 				scanner := rasterx.NewScannerGV(origW, origH, tex, tex.Bounds())
 				raster := rasterx.NewDasher(width, height, scanner)
@@ -85,12 +82,6 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 					return nil
 				}
 
-				if firstPassOriginal { // crop out the currently visible portion to avoid flicker when it sizes later
-					return imageSubset(tex, width, height, img.ScaleMode)
-				}
-
-				// this is used by our render code, so let's set it to the file aspect
-				aspects[img.Resource] = aspect
 				svgCachePut(img.Resource, tex, width, height)
 			}
 
@@ -105,42 +96,33 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 			return nil
 		}
 		origSize := pixels.Bounds().Size()
+		// this is used by our render code, so let's set it to the file aspect
+		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		// if the image specifies it should be original size we need at least that many pixels on screen
 		if img.FillMode == canvas.ImageFillOriginal {
 			if !checkImageMinSize(img, c, origSize.X, origSize.Y) {
-				return imageSubset(pixels, width, height, img.ScaleMode)
+				return nil
 			}
 		}
-		// this is used by our render code, so let's set it to the file aspect
-		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 
 		return scaleImage(pixels, width, height, img.ScaleMode)
 	case img.Image != nil:
 		origSize := img.Image.Bounds().Size()
+		// this is used by our render code, so let's set it to the file aspect
+		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		// if the image specifies it should be original size we need at least that many pixels on screen
 		if img.FillMode == canvas.ImageFillOriginal {
 			if !checkImageMinSize(img, c, origSize.X, origSize.Y) {
-				return imageSubset(img.Image, width, height, img.ScaleMode)
+				return nil
 			}
 		}
 
-		// this is used by our render code, so let's set it to the file aspect
-		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		return scaleImage(img.Image, width, height, img.ScaleMode)
 	default:
 		return image.NewNRGBA(image.Rect(0, 0, 1, 1))
 	}
 }
 
-// imageSubset returns a portion of an image based on pixel dimensions.
-// This is needed when we initially display an image with original dimensions that are still being calculated.
-func imageSubset(img image.Image, width, height int, scale canvas.ImageScale) image.Image {
-	sub := img.(interface {
-		SubImage(r image.Rectangle) image.Image
-	}).SubImage(image.Rect(0, 0, width, height))
-	return scaleImage(sub, width, height, scale)
-}
-
 func scaleImage(pixels image.Image, scaledW, scaledH int, scale canvas.ImageScale) image.Image {
 	pixW := fyne.Min(scaledW, pixels.Bounds().Dx()) // don't push more pixels than we have to
 	pixH := fyne.Min(scaledH, pixels.Bounds().Dy()) // the GL calls will scale this up on GPU.