op/paint: make every ImageOp unique

The gioui.org/commit/74407a50d598bfd27e8f8e48b6832cc5df04de77
added a NewImageOp constructor that always copies the supplied
image. It does that for two reasons:

First, the image.Image reference is used in the image=>texture
map of cached textures. Without a copy, we wouldn't detect a
modified image even if a new ImageOp was created.

Second, we don't want the program to touch the image while the GPU
is uploading it.

The second reason was removed in a previous change that blocks
FrameEvent.Frame until we're done with the operations, including
uploading images to the GPU.

The first reason is easily fixed by using a unique per ImageOp,
as pointed out by Alessandro Arzilli.

This change switches to using the unique key. Alessandro's patch
avoids the copy when possible.

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

internal/opconst/ops.go

@@ -34,7 +34,7 @@ const (
 	TypeTransformLen    = 1 + 4*2
 	TypeLayerLen        = 1
 	TypeRedrawLen       = 1 + 8
-	TypeImageLen        = 1 + 4*4
+	TypeImageLen        = 1
 	TypePaintLen        = 1 + 4*4
 	TypeColorLen        = 1 + 4
 	TypeAreaLen         = 1 + 1 + 4*4
@@ -74,8 +74,10 @@ func (t OpType) Size() int {
 
 func (t OpType) NumRefs() int {
 	switch t {
-	case TypeMacro, TypeImage, TypeKeyInput, TypePointerInput, TypeProfile:
+	case TypeMacro, TypeKeyInput, TypePointerInput, TypeProfile:
 		return 1
+	case TypeImage:
+		return 2
 	default:
 		return 0
 	}