fix(cocoa): real double buffering instead of fake glFlush swap

NSOpenGLPFADoubleBuffer was commented out, so DoubleBuffered:=True on the
control was silently ignored and the context stayed single-buffered.
LOpenGLSwapBuffers then only called glFlush(), leaving the front buffer on
screen mid-draw: frames tore and views could show partially rendered content.

Request the double-buffer pixel format when DoubleBuffered is set, and present
through NSOpenGLContext.flushBuffer. Falls back to glFlush() when no context
exists yet.

Note: this is a correctness fix, not a performance one -- measured on an M1 Pro
the app's CPU usage is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

glcocoanscontextex.pas

@@ -140,13 +140,23 @@ begin
 end;
 
 procedure LOpenGLSwapBuffers(Handle: HWND);
-//var
-//  View: TCocoaOpenGLView;  //TCocoaOpenGLView
+var
+  View: TCocoaOpenGLView;
+  Ctx: NSOpenGLContext;
 begin
   if Handle=0 then exit;
+  View:=TCocoaOpenGLView(Handle);
+  Ctx:=View.openGLContext;
+  if Ctx=nil then
+  begin
+    // single-buffered fallback (no context yet)
     glFlush();
-  // View:=TCocoaOpenGLView(Handle);
-  // View.nsGL.flushBuffer;
+    exit;
+  end;
+  // Present the back buffer. The old glFlush() left the single-buffered
+  // front buffer on screen mid-draw, so frames tore and the view could show
+  // partially rendered content.
+  Ctx.flushBuffer;
 end;
 
 function LOpenGLMakeCurrent(Handle: HWND): boolean;
@@ -283,8 +293,8 @@ var
     begin
       AddUInt32(NSOpenGLPFAAuxBuffers);  AddUInt32(AUXBuffers);
     end;
-    //if DoubleBuffered then //requires fix for nsGL
-    //   AddUInt32(NSOpenGLPFADoubleBuffer); //this doen't work with Lazarus
+    if DoubleBuffered then //present via real flushBuffer in LOpenGLSwapBuffers
+       AddUInt32(NSOpenGLPFADoubleBuffer);
     AddUInt32(NSOpenGLPFAMaximumPolicy); //allows future changes to make attributes more demanding, e.g. add multisampling
 
     AddUInt32(NSOpenGLPFANoRecovery); //see apple web page: "not generally useful" but might help with multisample