[Impeller] Use final cmd buffer to present drawable. (flutter/engine#46023)

If we're not running with a transaction then we don't need to block on waitUntilScheduled on any platform. I think this should always work, as the problems we had before were due to always using drawable present. But this helper method schedules the drawable presentation after the cmd buffer is scheduled - which is what we're doing with waitUntilScheduled anyway - just non blocking from our perspective.

Fixes https://github.com/flutter/flutter/issues/131520

engine/src/flutter/impeller/renderer/backend/metal/surface_mtl.mm

@@ -248,13 +248,22 @@ bool SurfaceMTL::Present() const {
   }
 
   if (drawable_) {
-    TRACE_EVENT0("flutter", "waitUntilScheduled");
     id<MTLCommandBuffer> command_buffer =
         ContextMTL::Cast(context.get())
             ->CreateMTLCommandBuffer("Present Waiter Command Buffer");
-    [command_buffer commit];
-    [command_buffer waitUntilScheduled];
-    [drawable_ present];
+    // If the threads have been merged, or there is a pending frame capture,
+    // then block on cmd buffer scheduling to ensure that the
+    // transaction/capture work correctly.
+    if ([[NSThread currentThread] isMainThread] ||
+        [[MTLCaptureManager sharedCaptureManager] isCapturing]) {
+      TRACE_EVENT0("flutter", "waitUntilScheduled");
+      [command_buffer commit];
+      [command_buffer waitUntilScheduled];
+      [drawable_ present];
+    } else {
+      [command_buffer presentDrawable:drawable_];
+      [command_buffer commit];
+    }
   }
 
   return true;