gpu: don't invoke GPU backend outside BeginFrame/EndFrame

The Metal backend is particularly sensitive because its staging buffers
are synchronized to BeginFrame.

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

gpu/gpu.go

@@ -403,7 +403,7 @@ func (g *gpu) Collect(viewport image.Point, frameOps *op.Ops) {
 	g.renderer.blitter.viewport = viewport
 	g.renderer.pather.viewport = viewport
 	g.drawOps.reset(g.cache, viewport)
-	g.drawOps.collect(g.ctx, g.cache, frameOps, viewport)
+	g.drawOps.collect(frameOps, viewport)
 	g.frameStart = time.Now()
 	if g.drawOps.profile && g.timers == nil && g.ctx.Caps().Features.Has(driver.FeatureTimers) {
 		g.timers = newTimers(g.ctx)
@@ -417,6 +417,7 @@ func (g *gpu) Frame(target RenderTarget) error {
 	viewport := g.renderer.blitter.viewport
 	defFBO := g.ctx.BeginFrame(target, g.drawOps.clear, viewport)
 	defer g.ctx.EndFrame()
+	g.drawOps.buildPaths(g.ctx)
 	for _, img := range g.drawOps.imageOps {
 		expandPathOp(img.path, img.clip)
 	}
@@ -810,7 +811,7 @@ func (d *drawOps) reset(cache *resourceCache, viewport image.Point) {
 	d.vertCache = d.vertCache[:0]
 }
 
-func (d *drawOps) collect(ctx driver.Device, cache *resourceCache, root *op.Ops, viewport image.Point) {
+func (d *drawOps) collect(root *op.Ops, viewport image.Point) {
 	clip := f32.Rectangle{
 		Max: f32.Point{X: float32(viewport.X), Y: float32(viewport.Y)},
 	}
@@ -821,6 +822,9 @@ func (d *drawOps) collect(ctx driver.Device, cache *resourceCache, root *op.Ops,
 		color: color.NRGBA{A: 0xff},
 	}
 	d.collectOps(&d.reader, state)
+}
+
+func (d *drawOps) buildPaths(ctx driver.Device) {
 	for _, p := range d.pathOps {
 		if v, exists := d.pathCache.get(p.pathKey); !exists || v.data.data == nil {
 			data := buildPath(ctx, p.pathVerts)