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>

op/paint/paint.go

@@ -20,9 +20,6 @@ import (
 // Note: the ImageOp may keep a reference to the backing image.
 // See NewImageOp for details.
 type ImageOp struct {
-	// Rect is the section if the backing image to use.
-	Rect image.Rectangle
-
 	uniform bool
 	color   color.RGBA
 	src     *image.RGBA
@@ -74,7 +71,6 @@ func NewImageOp(src image.Image) ImageOp {
 		bounds := src.Bounds()
 		if bounds.Min == (image.Point{}) && src.Stride == bounds.Dx()*4 {
 			return ImageOp{
-				Rect:   src.Bounds(),
 				src:    src,
 				handle: new(int),
 			}
@@ -88,7 +84,6 @@ func NewImageOp(src image.Image) ImageOp {
 	})
 	draw.Draw(dst, dst.Bounds(), src, src.Bounds().Min, draw.Src)
 	return ImageOp{
-		Rect:   dst.Bounds(),
 		src:    dst,
 		handle: new(int),
 	}
@@ -110,11 +105,6 @@ func (i ImageOp) Add(o *op.Ops) {
 	}
 	data := o.Write(opconst.TypeImageLen, i.src, i.handle)
 	data[0] = byte(opconst.TypeImage)
-	bo := binary.LittleEndian
-	bo.PutUint32(data[1:], uint32(i.Rect.Min.X))
-	bo.PutUint32(data[5:], uint32(i.Rect.Min.Y))
-	bo.PutUint32(data[9:], uint32(i.Rect.Max.X))
-	bo.PutUint32(data[13:], uint32(i.Rect.Max.Y))
 }
 
 func (c ColorOp) Add(o *op.Ops) {