Limit number of parallel frames being painted (#365)

In #67, we started painting frames in parallel instead of serially, in
order to fully utilize available CPU.

But we did this without any limit. So if a `Root` had hundreds of frames,
we would attempt to paint all of them in parallel. This would consume
huge amounts of memory by allocating hundreds or thousands of image
buffers.

Instead, we now limit parallelism to the number of CPU's.

render/root.go

@@ -3,6 +3,7 @@ package render
 import (
 	"image"
 	"image/color"
+	"runtime"
 	"sync"
 
 	"github.com/tidbyt/gg"
@@ -38,18 +39,51 @@ type Root struct {
 	Child  Widget `starlark:"child,required"`
 	Delay  int32  `starlark:"delay"`
 	MaxAge int32  `starlark:"max_age"`
+
+	maxParallelFrames int
+}
+
+type RootPaintOption func(*Root)
+
+// WithMaxParallelFrames sets the maximum number of frames that will
+// be painted in parallel.
+//
+// By default, only `runtime.NumCPU()` frames are painted in parallel.
+// Higher parallelism consumes more memory, and doesn't usually make
+// sense since painting is CPU-bouond.
+func WithMaxParallelFrames(max int) RootPaintOption {
+	return func(r *Root) {
+		r.maxParallelFrames = max
+	}
 }
 
 // Paint renders the child widget onto the frame. It doesn't do
 // any resizing or alignment.
-func (r Root) Paint(solidBackground bool) []image.Image {
+func (r Root) Paint(solidBackground bool, opts ...RootPaintOption) []image.Image {
+	for _, opt := range opts {
+		opt(&r)
+	}
+
 	numFrames := r.Child.FrameCount()
 	frames := make([]image.Image, numFrames)
 
+	parallelism := r.maxParallelFrames
+	if parallelism <= 0 {
+		parallelism = runtime.NumCPU()
+	}
+
 	var wg sync.WaitGroup
+	sem := make(chan bool, parallelism)
 	for i := 0; i < numFrames; i++ {
 		wg.Add(1)
+		sem <- true
+
 		go func(i int) {
+			defer func() {
+				<-sem
+				wg.Done()
+			}()
+
 			dc := gg.NewContext(DefaultFrameWidth, DefaultFrameHeight)
 			if solidBackground {
 				dc.SetColor(color.Black)
@@ -60,7 +94,6 @@ func (r Root) Paint(solidBackground bool) []image.Image {
 			r.Child.Paint(dc, image.Rect(0, 0, DefaultFrameWidth, DefaultFrameHeight), i)
 			dc.Pop()
 			frames[i] = dc.Image()
-			wg.Done()
 		}(i)
 	}