Add 2D MSAA support for compatibility renderer

[troybonneau]

Summary

• add MSAA renderbuffer handling to the GLES3 texture storage so 2D render targets can store multi-sample data and resolve it on demand
• update the canvas renderer to render through the MSAA FBO, copy the existing contents when needed, and request resolves appropriately
• only warn about unused 2D MSAA when no 2D content was rendered in the viewport pass

Testing

• python3 -m SCons -j2 platform=linuxbsd target=editor (incomplete – build interrupted)

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https://chatgpt.com/codex/tasks/task_e_68ca5f5e79c8832286ec501769d075da