gpu,widget,op/paint,gpu: remove support for ImageOp.Rect

This is effectively a revert of commit gioui.org/commit/69dfd2e3a5541.

ImageOp.Rect is the wrong abstraction; it implies a clipping operation that is
better handled by package clip.

API change. Uses of ImageOp.Rect should apply a clip.Rect before the PaintOp,
or use image.RGBA.SubImage (or similar).

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/image.go

@@ -28,7 +28,7 @@ func (im Image) Layout(gtx layout.Context) layout.Dimensions {
 	if scale == 0 {
 		scale = 160.0 / 72.0
 	}
-	size := im.Src.Rect.Size()
+	size := im.Src.Size()
 	wf, hf := float32(size.X), float32(size.Y)
 	w, h := gtx.Px(unit.Dp(wf*scale)), gtx.Px(unit.Dp(hf*scale))
 	cs := gtx.Constraints