internal/egl: fix loadEGL caching error on Windows

loadEGL used sync.Once incorrectly: the error returned by
loadDLLs was assigned to a local variable inside loadEGL,
so on the second call Do would not run and the
function would return nil even though the DLLs were never
loaded. This caused a nil pointer dereference when callers
proceeded to use _eglGetDisplay and other uninitialized
function pointers.

Fix by replacing the sync.Once with sync.OnceValue, which
correctly caches the return value of loadDLLs across all
calls.

Signed-off-by: Kevin Yuan <farproc@gmail.com>

internal/egl/egl_windows.go

@@ -41,14 +41,10 @@ var (
 	_eglWaitClient          *syscall.Proc
 )
 
-var loadOnce sync.Once
+var loadOnce = sync.OnceValue(loadDLLs)
 
 func loadEGL() error {
-	var err error
-	loadOnce.Do(func() {
-		err = loadDLLs()
-	})
-	return err
+	return loadOnce()
 }
 
 func loadDLLs() error {