app: avoid relocking contexts every frame

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

app/gl_ios.go

@@ -115,9 +115,6 @@ 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

@@ -169,6 +169,7 @@ 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