Always enable OpenGL fallback backend, fix --renderer=gl only working together with WGPU_BACKEND env-var

[Wumpf]

What

Previously, we made the GL backend explicit opt-in (and accidentally only via the wgpu env var, not solely via commandline). But I don't see any reason for this as wgpu will (well, should) always prefer Vulkan when available.

• Fixes #5968
  • Reportedly there's still sometimes issues with he GL backend, but this is captured now in Improve error message (and investigate causes) for unsuitable GL adapter #6581

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