all: serialize ops

Pros:
- Much less per-frame garbage
- Allow future preprocessing of ops while building it
- Much fewer interface calls and pointer chasing
- Allow future serialization of ops for remote rendering

Cons:
- Slightly clumsier API

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

ui/draw/path.go

@@ -3,16 +3,17 @@
 package draw
 
 import (
+	"encoding/binary"
 	"math"
 
 	"gioui.org/ui"
 	"gioui.org/ui/f32"
+	"gioui.org/ui/internal/ops"
 	"gioui.org/ui/internal/path"
 )
 
 type OpClip struct {
 	Path *Path
-	Op   ui.Op
 }
 
 type Path struct {
@@ -33,11 +34,25 @@ func (p *Path) Data() interface{} {
 	return p.data
 }
 
-func (p OpClip) ChildOp() ui.Op {
-	return p.Op
+func (c OpClip) Add(o *ui.Ops) {
+	data := make([]byte, ops.TypeClipLen)
+	data[0] = byte(ops.TypeClip)
+	bo := binary.LittleEndian
+	ref := o.Ref(c.Path)
+	bo.PutUint32(data[1:], uint32(ref))
+	o.Write(data)
 }
 
-func (p OpClip) ImplementsOp() {}
+func (c *OpClip) Decode(d []byte, refs []interface{}) {
+	bo := binary.LittleEndian
+	if ops.OpType(d[0]) != ops.TypeClip {
+		panic("invalid op")
+	}
+	ref := int(bo.Uint32(d[1:]))
+	*c = OpClip{
+		Path: refs[ref].(*Path),
+	}
+}
 
 // MoveTo moves the pen to the given position.
 func (p *PathBuilder) Move(to f32.Point) {
@@ -265,3 +280,5 @@ func (p *PathBuilder) Path() *Path {
 	}
 	return data
 }
+
+func (p OpClip) ImplementsOp() {}