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>

internal/opconst/ops.go

@@ -35,7 +35,7 @@ const (
 	TypeTransformLen      = 1 + 4*6
 	TypeLayerLen          = 1
 	TypeRedrawLen         = 1 + 8
-	TypeImageLen          = 1 + 4*4
+	TypeImageLen          = 1
 	TypePaintLen          = 1 + 4*4
 	TypeColorLen          = 1 + 4
 	TypeLinearGradientLen = 1 + 8*2 + 4*2