internal/stroke, gpu: reuse slice when splitting cubics

When building GPU vertices from paths, we call stroke.SplitCubic once
per OpCubic. Before this change, each call to stroke.SplitCubic would
allocate a slice, which we would only use to iterate over.

This allocation can be easily avoided by reusing the slice. We can
conveniently store it in gpu.quadSplitter.

In a real application that renders hundreds of paths with tens of
rounded rectangles per path, this saved roughly 4500 allocations (or 1
MB worth) per frame.

Signed-off-by: Dominik Honnef <dominik@honnef.co>

gpu/clip.go

@@ -12,6 +12,9 @@ type quadSplitter struct {
 	bounds  f32.Rectangle
 	contour uint32
 	d       *drawOps
+
+	// scratch space used by calls to stroke.SplitCubic
+	scratch []stroke.QuadSegment
 }
 
 func encodeQuadTo(data []byte, meta uint32, from, ctrl, to f32.Point) {