app: let drivers control Window directly

Before this change, Window driver callbacks would all go through
channels to be processed by Window.run. However, Window.run may call
into the driver, which again may invoke a Window callback. These
re-entrant calls have been a source of deadlocks and subtle errors,
resulting in increasingly complex channel logic. This change eliminates
the goroutine split between Window and the driver, allowing callbacks
between Window and the driver without restrictions.

The goroutine split between Window and the driver is historical and
was meant to tame the complicated callback logic of drivers into a nice
for-select loop. However, the complexity isn't worth the gain, and there is
no concurrency concerns because there is always a 1:1 correspondance between
a driver goroutine and its Window object.

Fixes: https://todo.sr.ht/~eliasnaur/gio/329
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os_android.go

@@ -165,6 +165,7 @@ type window struct {
 		hoverID router.SemanticID
 		rootID  router.SemanticID
 		focusID router.SemanticID
+		diffs   []router.SemanticID
 	}
 }
 
@@ -856,12 +857,8 @@ func (w *window) draw(env *C.JNIEnv, sync bool) {
 			w.semantic.rootID = newR
 			callVoidMethod(env, w.view, gioView.sendA11yChange, jvalue(w.virtualIDFor(newR)))
 		}
-		diffs := w.callbacks.RequestSemanticDiffs()
-		for {
-			id := <-diffs
-			if id == 0 {
-				break
-			}
+		w.semantic.diffs = w.callbacks.AppendSemanticDiffs(w.semantic.diffs[:0])
+		for _, id := range w.semantic.diffs {
 			callVoidMethod(env, w.view, gioView.sendA11yChange, jvalue(w.virtualIDFor(id)))
 		}
 	}