Fix background(img) handling in WEBGL mode in RendererGL

[Helenbeyorn]

This PR adds support for using background(p5.Image) in WEBGL mode, addressing issue #7917.
What was done:

Modified p5.RendererGL.js to check if the argument is an instance of p5.Image and render it using image().

Confirmed that image background renders properly in WEBGL when served via a local server (python3 -m http.server 5500).

Notes:

The image does not show up when using VS Code Live Server, but this seems to be an environment-specific issue.

Feedback is welcome if there are better integration points for this in the renderer.

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