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/window.go

@@ -70,6 +70,7 @@ type Context interface {
 	API() gpu.API
 	Present() error
 	MakeCurrent() error
+	ReleaseCurrent()
 	Refresh() error
 	Release()
 	Lock()