app/internal/window: rename "backends" to "drivers"

"driver" is the new more specific name for gpu backends.

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

app/internal/window/d3d11_windows.go

@@ -24,7 +24,7 @@ type d3d11Context struct {
 const debug = false
 
 func init() {
-	backends = append(backends, gpuAPI{
+	drivers = append(drivers, gpuAPI{
 		priority: 1,
 		initializer: func(w *window) (Context, error) {
 			hwnd, _, _ := w.HWND()