op/clip: remove complex stroke support

In a discussion with Raph Levien, the author of our compute renderer
implementation, it became clear to me that it's not at all certain that
complex strokes will ever be efficiently supported by a GPU renderer.

At the same time, the machinery for converting a complex stroke to a
GPU-friendly outline has a significant maintenance cost. Further, it is
surprising to users that complex strokes are significantly slower and
allocate memory.

This change removes support for complex strokes, leaving only
round-capped, round-joined strokes supported by the compute renderer.
The default renderer still converts all strokes to outline, but it also
caches the result.

This is an API change. The complex stroke conversion code has been moved
to the external gioui.org/x/stroke package, with a similar API.

Updats gio#282 (Inkeliz brought up the allocation issue)

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

gpu/compute.go

@@ -28,7 +28,6 @@ import (
 	"gioui.org/internal/scene"
 	"gioui.org/layout"
 	"gioui.org/op"
-	"gioui.org/op/clip"
 	"gioui.org/shader"
 	"gioui.org/shader/gio"
 	"gioui.org/shader/piet"
@@ -236,10 +235,10 @@ type encoderState struct {
 // clipKey completely describes a clip operation (along with its path) and is appropriate
 // for hashing and equality checks.
 type clipKey struct {
-	bounds   f32.Rectangle
-	stroke   clip.StrokeStyle
-	relTrans f32.Affine2D
-	pathHash uint64
+	bounds      f32.Rectangle
+	strokeWidth float32
+	relTrans    f32.Affine2D
+	pathHash    uint64
 }
 
 // paintKey completely defines a paint operation. It is suitable for hashing and
@@ -1683,7 +1682,7 @@ func (c *opsCollector) reset() {
 	c.layers = c.layers[:0]
 }
 
-func (c *collector) addClip(state *encoderState, viewport, bounds f32.Rectangle, path []byte, key ops.Key, hash uint64, stroke clip.StrokeStyle, push bool) {
+func (c *collector) addClip(state *encoderState, viewport, bounds f32.Rectangle, path []byte, key ops.Key, hash uint64, strokeWidth float32, push bool) {
 	// Rectangle clip regions.
 	if len(path) == 0 && !push {
 		// If the rectangular clip region contains a previous path it can be discarded.
@@ -1713,10 +1712,10 @@ func (c *collector) addClip(state *encoderState, viewport, bounds f32.Rectangle,
 		intersect: intersect,
 		push:      push,
 		clipKey: clipKey{
-			bounds:   bounds,
-			relTrans: state.relTrans,
-			stroke:   stroke,
-			pathHash: hash,
+			bounds:      bounds,
+			relTrans:    state.relTrans,
+			strokeWidth: strokeWidth,
+			pathHash:    hash,
 		},
 	})
 	state.clip = &c.clipStates[len(c.clipStates)-1]
@@ -1742,9 +1741,9 @@ func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textur
 			key  ops.Key
 			hash uint64
 		}
-		str clip.StrokeStyle
+		strWidth float32
 	)
-	c.addClip(&state, fview, fview, nil, ops.Key{}, 0, clip.StrokeStyle{}, false)
+	c.addClip(&state, fview, fview, nil, ops.Key{}, 0, 0, false)
 	for encOp, ok := r.Decode(); ok; encOp, ok = r.Decode() {
 		switch ops.OpType(encOp.Data[0]) {
 		case ops.TypeProfile:
@@ -1763,7 +1762,7 @@ func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textur
 			state.t = st.t
 			state.relTrans = st.relTrans
 		case ops.TypeStroke:
-			str = decodeStrokeOp(encOp.Data)
+			strWidth = decodeStrokeOp(encOp.Data)
 		case ops.TypePath:
 			hash := bo.Uint64(encOp.Data[1:])
 			encOp, ok = r.Decode()
@@ -1776,9 +1775,9 @@ func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textur
 		case ops.TypeClip:
 			var op clipOp
 			op.decode(encOp.Data)
-			c.addClip(&state, fview, op.bounds, pathData.data, pathData.key, pathData.hash, str, op.push)
+			c.addClip(&state, fview, op.bounds, pathData.data, pathData.key, pathData.hash, strWidth, op.push)
 			pathData.data = nil
-			str = clip.StrokeStyle{}
+			strWidth = 0
 		case ops.TypePopClip:
 			for {
 				push := state.clip.push
@@ -1806,7 +1805,7 @@ func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textur
 			if paintState.matType == materialTexture {
 				// Clip to the bounds of the image, to hide other images in the atlas.
 				bounds := paintState.image.src.Bounds()
-				c.addClip(&paintState, fview, layout.FRect(bounds), nil, ops.Key{}, 0, clip.StrokeStyle{}, false)
+				c.addClip(&paintState, fview, layout.FRect(bounds), nil, ops.Key{}, 0, 0, false)
 			}
 			intersect := paintState.clip.intersect
 			if intersect.Empty() {
@@ -2101,9 +2100,9 @@ func encodeOp(viewport image.Point, absOff image.Point, enc *encoder, texOps []t
 	enc.transform(inv)
 	for i := len(op.clipStack) - 1; i >= 0; i-- {
 		cl := op.clipStack[i]
-		if str := cl.state.stroke; str.Width > 0 {
+		if w := cl.state.strokeWidth; w > 0 {
 			enc.fillMode(scene.FillModeStroke)
-			enc.lineWidth(str.Width)
+			enc.lineWidth(w)
 			fillMode = scene.FillModeStroke
 		} else if fillMode != scene.FillModeNonzero {
 			enc.fillMode(scene.FillModeNonzero)