app: relock contexts after refresh

This fixes an Android regression introduced by

  3e601e73c4
  app: optimize window context locking

On Android, that change can leave the app window visible but unrendered:
the screen stays black instead of drawing the UI.

The regression is in app/window.go: Refresh can update the underlying
surface state without guaranteeing a subsequent Lock before rendering.

That is a problem for Android, where Refresh and Lock have distinct
roles:

- androidContext.Refresh stages the native window for the EGL surface
- androidContext.Lock performs CreateSurface, when needed, and MakeCurrent

In other words, after Refresh, Android may require another Lock before
GPU work can proceed correctly.

This patch keeps explicit locking, but avoids unconditional per-frame
Lock/Unlock calls. It adds a small amount of state to Window:

- ctxNeedsLock is set when creating a context
- ctxNeedsLock is set after Refresh
- ctxNeedsLock is set after explicit unlock/release paths
- Lock is called before GPU work only when ctxNeedsLock is true

That keeps the optimized steady-state path while restoring the required
Refresh -> Lock sequencing.

I also added a regression test for validateAndProcess that checks:

- Refresh forces a re-lock before drawing/present
- steady-state frames do not re-lock redundantly

Verification:
- go test ./app

Signed-off-by: Joe Julian <me@joejulian.name>

app/gl_ios.go

@@ -115,6 +115,9 @@ func (c *context) Unlock() {
 }
 
 func (c *context) Refresh() error {
+	if C.gio_makeCurrent(c.ctx) == 0 {
+		return errors.New("[EAGLContext setCurrentContext] failed")
+	}
 	if !c.init {
 		c.init = true
 		c.frameBuffer = c.c.CreateFramebuffer()

app/os.go

@@ -169,7 +169,6 @@ type context interface {
 	API() gpu.API
 	RenderTarget() (gpu.RenderTarget, error)
 	Present() error
-	// Refresh assumes the context is already locked and current.
 	Refresh() error
 	Release()
 	Lock() error