fix(demos): fix video-element webcam moveTo error on Fabric 6

[kiranparajuli589]

Problem

The video-element demo throws TypeError: webcam.moveTo is not a function
after the user grants camera permission, leaving the webcam preview black.

fabric.Object#moveTo(index) was removed in Fabric 6 (per-object stacking
moved to canvas methods). The demo runs under two Fabric versions:

• v5.2.1 on the live demo page (_layouts/demo.html loads lib/fabric.js) — moveTo works.
• v6 via the "Open in CodePen" button (_includes/codepen.html loads
  fabric@latest) and the new fabricjs.com — moveTo throws.

Fix

Feature-detect the API so the demo works under both versions: use
canvas.sendObjectToBack(webcam) when available (Fabric 6+), otherwise fall
back to webcam.moveTo(0) (Fabric 5). sendObjectToBack is the exact v6
equivalent of moveTo(0).

Only posts/demos/_posts/2014-10-16-video-element.html is touched.

Testing

• Local (jekyll serve, v5.2.1): open /demos/video-element/, grant camera →
  no console error, webcam renders behind the sample video.
• CodePen (v6 via unpkg): same demo, grant camera → no moveTo TypeError.

Signed-off-by: Kiran Parajuli 39373750+kiranparajuli589@users.noreply.github.com