exp/audio: APIs should not use Context pointer type

This CL is a followup of CL/11315 where it is suggested to remove
Context pointer types from the signatures.

Change-Id: Ice68bfabad0e345f7d93102257a99402be8760b5
Reviewed-on: https://go-review.googlesource.com/11560
Reviewed-by: Nigel Tao <nigeltao@golang.org>

exp/audio/al/alc.go

@@ -54,27 +54,25 @@ func OpenDevice() error {
 }
 
 // CreateContext creates a new context.
-func CreateContext() (*Context, error) {
+func CreateContext() (Context, error) {
 	ctx := alcCreateContext(device, nil)
 	if ctx == nil {
-		return nil, errors.New("al: cannot create a new context")
+		return Context{}, errors.New("al: cannot create a new context")
 	}
-	return &Context{ptr: ctx}, nil
+	return Context{ptr: ctx}, nil
 }
 
 // Destroy destroys the context and frees the underlying resources.
 // The current context cannot be destroyed. Use MakeContextCurrent
 // to make a new context current or use nil.
-func (c *Context) Destroy() {
+func (c Context) Destroy() {
 	alcDestroyContext(c.ptr)
 }
 
-// MakeContextCurrent makes the given context current. The current
-// context can be nil.
-func MakeContextCurrent(c *Context) bool {
-	if c == nil {
-		return alcMakeContextCurrent(nil)
-	}
+// MakeContextCurrent makes the given context current. Calls to
+// MakeContextCurrent can accept a zero value Context to allow
+// a program not to have an active current context.
+func MakeContextCurrent(c Context) bool {
 	return alcMakeContextCurrent(c.ptr)
 }
 

exp/audio/audio.go

@@ -104,7 +104,7 @@ var (
 	mu sync.Mutex // guards ctx
 
 	// ctx is the shared al.Context used by multiple Player instances.
-	ctx *al.Context
+	ctx al.Context
 )
 
 type track struct {
@@ -170,9 +170,10 @@ func createContext() error {
 	mu.Lock()
 	defer mu.Unlock()
 
-	if ctx != nil {
+	if ctx != (al.Context{}) {
 		return nil
 	}
+
 	var err error
 	ctx, err = al.CreateContext()
 	if err != nil {