gpu,gpu/headless,app/internal/wm: add explicit RenderTarget API

Both the OpenGL and the Direct3D API are stateful and gpu.GPU renders to
the render target current when Frame is called.

Modern GPU API such as Metal don't have a concept of a current render
target, and the target even changes each frame.

Add RenderTarget and add an explicit target argument to GPU.Frame as
well as the underlying driver.Device.BeginFrame.

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

app/loop.go

@@ -100,7 +100,7 @@ func (l *renderLoop) renderLoop(ctx wm.Context) error {
 				g.Collect(frame.viewport, frame.ops)
 				// Signal that we're done with the frame ops.
 				l.ack <- struct{}{}
-				res.err = g.Frame()
+				res.err = g.Frame(ctx.RenderTarget())
 				if res.err == nil {
 					res.err = ctx.Present()
 				}