app,app/internal/wm: release OpenGL context after use

Otherwise, making a context current on another thread may result in
an EGL_BAD_ACCESS error.

Fixes gio#248

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

app/internal/wm/gl_ios.go

@@ -139,6 +139,10 @@ func (c *context) MakeCurrent() error {
 	return nil
 }
 
+func (c *context) ReleaseCurrent() {
+	C.gio_makeCurrent(0)
+}
+
 func (w *window) NewContext() (Context, error) {
 	return newContext(w)
 }