Fix iOS screenshot API to use dispatch_sync and proper memory management

The initial implementation used dispatch_async which caused a NullPointerException
because Java method calls require proper thread context. When using dispatch_async,
the calling thread's context is not maintained in the async block.

Changes:
- Changed dispatch_async to dispatch_sync to maintain thread context for Java calls
- Added POOL_BEGIN() at the start of the dispatch block for proper autorelease pool
- Added POOL_END() before early returns and at the end of the block

This follows the same pattern used throughout the codebase for operations that
capture images and call back to Java (e.g., createPeerImage at line 2076).

Author: claude

Ports/iOSPort/nativeSources/IOSNative.m

@@ -5212,15 +5212,18 @@ void com_codename1_impl_ios_IOSNative_updatePersonWithRecordID___int_com_codenam
 }
 
 void com_codename1_impl_ios_IOSNative_screenshot__(CN1_THREAD_STATE_MULTI_ARG JAVA_OBJECT instanceObject) {
-    dispatch_async(dispatch_get_main_queue(), ^{
+    dispatch_sync(dispatch_get_main_queue(), ^{
+        POOL_BEGIN();
         UIView *view = [CodenameOne_GLViewController instance].view;
         UIImage *img = cn1_captureView(view);
         if (!img) {
+            POOL_END();
             return;
         }
 
         NSData *png = UIImagePNGRepresentation(img);
         if (!png) {
+            POOL_END();
             return;
         }
 
@@ -5231,6 +5234,7 @@ void com_codename1_impl_ios_IOSNative_screenshot__(CN1_THREAD_STATE_MULTI_ARG JA
         memcpy((JAVA_ARRAY_BYTE*)((JAVA_ARRAY)byteArr)->data, (const jbyte*)[png bytes], len);
 
         com_codename1_impl_ios_IOSImplementation_onScreenshot___byte_1ARRAY(CN1_THREAD_GET_STATE_PASS_ARG byteArr);
+        POOL_END();
     });
 }