gpui: Webgl backend (#62165)

Currently gpui only supports webgpu as a backend when compiling for the
web. However, webgpu support on Linux browsers is spotty (without
setting experimental flags):
- chromium has is enabled for nvidia and recent intel chips
- firefox does not enable it

This PR adds a WebGL backend, which is much more widely supported

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Release Notes:

- N/A or Added/Fixed/Improved ...

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Co-authored-by: Lukas Wirth <me@lukaswirth.dev>