fix: Set framebuffer to null only when canvas source matches main canvas

[ShukantPal]

Description of change

In my work on @pixi-essentials/svg, I was seeing that copying from a canvas to a render texture was not working using renderer.renderTarget.copyToTexture, even though the signature accepts canvas sources. I traced it to PixiJS assigning the canvas texture a null framebuffer on binding to WebGL. This is because it incorrectly assumes that a canvas input must be the main canvas.

https://developer.mozilla.org/en-US/docs/Web/API/WebGLRenderingContext/bindFramebuffer

A WebGLFramebuffer object to bind, or null for binding the HTMLCanvasElement or OffscreenCanvas object associated with the rendering context.

Pre-Merge Checklist

• Tests and/or benchmarks are included
• Documentation is changed or added
• Lint process passed (npm run lint)
• Tests passed (npm run test)

[github-actions]

Your PR title doesn't match the required format. The title should be in this format:

chore: update Text docs
fix: text not rendering
feat: add new feature to Text
breaking: remove Text#resolution 

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit ed56504:

Sandbox	Source
pixi.js-sandbox	Configuration

[GoodBoyDigital]

thanks @ShukantPal 👍