gpu: Merge GPU.Collect and GPU.Frame

There's no meaningful reason to have them separate. The intention was to
enable rendering concurrent with other processing, but that's gaining
framerate at the expense of input latency and complicating ImageOp
semantics.

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

gpu/headless/headless.go

@@ -109,8 +109,7 @@ func (w *Window) Release() {
 func (w *Window) Frame(frame *op.Ops) error {
 	return contextDo(w.ctx, func() error {
 		w.gpu.Clear(color.NRGBA{})
-		w.gpu.Collect(w.size, frame)
-		return w.gpu.Frame(driver.RenderTarget(w.fbo))
+		return w.gpu.Frame(frame, driver.RenderTarget(w.fbo), w.size)
 	})
 }