dnd kitten: remove empty-payload EOF QueueDnDData calls for thumbnails

The empty-payload second QueueDnDData call in set_drag_image() and
set_drag_image_text() triggered base64_decode_stream on an already-
completed (padded) stream, which returns failure. This caused cancel_drag
→ drag_free_offer to run, destroying all drag state before drag_start
was called from t=P:x=-1. As a result, drag_remote_file_data found no
item with requested_remote_files set and aborted with EINVAL.

The empty EOF call was never needed: drag_add_image only accumulates
decoded bytes; the actual image processing happens later in drag_start.
This mirrors the same fix already applied to the MIME pre-send path.

Agent-Logs-Url: https://github.com/kovidgoyal/kitty/sessions/8bb89dc9-fd72-41c5-892b-2a15c658b313

Co-authored-by: kovidgoyal <1308621+kovidgoyal@users.noreply.github.com>

Author: Copilot

kittens/dnd/drag.go

@@ -116,10 +116,7 @@ func (dnd *dnd) set_drag_image_text() (err error) {
 	if icon == "" {
 		icon = strings.TrimSpace("󰮐 ")
 	}
-	cmd := DC{Type: 'p', X: -1, Xp: 6, Yp: 1, Payload: []byte(icon)}
-	dnd.lp.QueueDnDData(cmd)
-	cmd.Payload = nil
-	dnd.lp.QueueDnDData(cmd)
+	dnd.lp.QueueDnDData(DC{Type: 'p', X: -1, Xp: 6, Yp: 1, Payload: []byte(icon)})
 	return nil
 }
 
@@ -151,12 +148,9 @@ func (dnd *dnd) set_drag_image() (err error) {
 	} else {
 		pix = imaging.AsRGBAData8(img)
 	}
-	cmd := DC{
+	dnd.lp.QueueDnDData(DC{
 		Type: 'p', X: -1, Y: utils.IfElse(num_channels == 3, 24, 32), Xp: img.Bounds().Dx(), Yp: img.Bounds().Dy(),
-		Payload: pix}
-	dnd.lp.QueueDnDData(cmd)
-	cmd.Payload = nil
-	dnd.lp.QueueDnDData(cmd)
+		Payload: pix})
 	return nil
 }