use point pool

For internal operations (anything using getAndClearPoints), there's a
pretty good chance that the operation will repeatedly invoke something
like fillPolygon(), meaning that it needs to push "a few" points
and then invoke something that uses those points.

So, we add a slice for containing spare slices of points, and on the
way out of each such function, shove the current imd.points (as used
inside that function) onto a stack, and set imd.points to [0:0] of
the thing it was called with.

Performance goes from 11-13fps to 17-18fps on my test case.

imdraw/imdraw.go

@@ -52,6 +52,7 @@ type IMDraw struct {
 	EndShape  EndShape
 
 	points []point
+	pool   [][]point
 	matrix pixel.Matrix
 	mask   pixel.RGBA
 
@@ -109,7 +110,7 @@ func (imd *IMDraw) Clear() {
 //
 // This does not affect matrix and color mask set by SetMatrix and SetColorMask.
 func (imd *IMDraw) Reset() {
-	imd.points = nil
+	imd.points = imd.points[:0]
 	imd.Color = pixel.Alpha(1)
 	imd.Picture = pixel.ZV
 	imd.Intensity = 0
@@ -256,10 +257,22 @@ func (imd *IMDraw) EllipseArc(radius pixel.Vec, low, high, thickness float64) {
 
 func (imd *IMDraw) getAndClearPoints() []point {
 	points := imd.points
+	// use one of the existing pools so we don't reallocate as often
+	if len(imd.pool) > 0 {
+		pos := len(imd.pool) - 1
+		imd.points = imd.pool[pos]
+		imd.pool = imd.pool[0:pos]
+	} else {
 		imd.points = nil
+	}
 	return points
 }
 
+func (imd *IMDraw) restorePoints(points []point) {
+	imd.pool = append(imd.pool, imd.points)
+	imd.points = points[:0]
+}
+
 func (imd *IMDraw) applyMatrixAndMask(off int) {
 	for i := range (*imd.tri)[off:] {
 		(*imd.tri)[off+i].Position = imd.matrix.Project((*imd.tri)[off+i].Position)
@@ -271,6 +284,7 @@ func (imd *IMDraw) fillRectangle() {
 	points := imd.getAndClearPoints()
 
 	if len(points) < 2 {
+		imd.restorePoints(points)
 		return
 	}
 
@@ -302,12 +316,14 @@ func (imd *IMDraw) fillRectangle() {
 
 	imd.applyMatrixAndMask(off)
 	imd.batch.Dirty()
+	imd.restorePoints(points)
 }
 
 func (imd *IMDraw) outlineRectangle(thickness float64) {
 	points := imd.getAndClearPoints()
 
 	if len(points) < 2 {
+		imd.restorePoints(points)
 		return
 	}
 
@@ -323,12 +339,14 @@ func (imd *IMDraw) outlineRectangle(thickness float64) {
 		imd.pushPt(pixel.V(b.pos.X, a.pos.Y), mid)
 		imd.polyline(thickness, true)
 	}
+	imd.restorePoints(points)
 }
 
 func (imd *IMDraw) fillPolygon() {
 	points := imd.getAndClearPoints()
 
 	if len(points) < 3 {
+		imd.restorePoints(points)
 		return
 	}
 
@@ -346,6 +364,7 @@ func (imd *IMDraw) fillPolygon() {
 
 	imd.applyMatrixAndMask(off)
 	imd.batch.Dirty()
+	imd.restorePoints(points)
 }
 
 func (imd *IMDraw) fillEllipseArc(radius pixel.Vec, low, high float64) {
@@ -387,6 +406,7 @@ func (imd *IMDraw) fillEllipseArc(radius pixel.Vec, low, high float64) {
 		imd.applyMatrixAndMask(off)
 		imd.batch.Dirty()
 	}
+	imd.restorePoints(points)
 }
 
 func (imd *IMDraw) outlineEllipseArc(radius pixel.Vec, low, high, thickness float64, doEndShape bool) {
@@ -485,12 +505,14 @@ func (imd *IMDraw) outlineEllipseArc(radius pixel.Vec, low, high, thickness floa
 			}
 		}
 	}
+	imd.restorePoints(points)
 }
 
 func (imd *IMDraw) polyline(thickness float64, closed bool) {
 	points := imd.getAndClearPoints()
 
 	if len(points) == 0 {
+		imd.restorePoints(points)
 		return
 	}
 	if len(points) == 1 {
@@ -591,4 +613,5 @@ func (imd *IMDraw) polyline(thickness float64, closed bool) {
 			imd.fillEllipseArc(pixel.V(thickness/2, thickness/2), normal.Angle(), normal.Angle()-math.Pi)
 		}
 	}
+	imd.restorePoints(points)
 }