Fix main canvas clipping getting applied to framebuffers

[davepagurek]

Resolves #6989

Changes:

• Makes sure the WebGL stencil check, which is what applies the clipping, is updated both when you start drawing to a framebuffer as well as when you stop (previously, it only did this when you stop)

Screenshots of the change:

The code in the issue now produces a red canvas in Chrome as expected:

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated