Always enable OpenGL fallback backend, fix --renderer=gl only working together with WGPU_BACKEND env-var (#6582)

### 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](#5968)
* Reportedly there's still sometimes issues with he GL backend, but this
is captured now in #6581

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6582?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6582?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/6582)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

Author: Wumpf

crates/re_renderer/src/config.rs

@@ -223,16 +223,18 @@ pub struct RenderContextConfig {
 /// Other backend might work as well, but lack of support isn't regarded as a bug.
 pub fn supported_backends() -> wgpu::Backends {
     if cfg!(native) {
-        // Native.
-        // Only use Vulkan & Metal unless explicitly told so since this reduces surfaces and thus surprises.
-        //
-        // Bunch of cases where it's still useful to switch though:
-        // * Some Windows VMs only provide DX12 drivers, observed with Parallels on Apple Silicon
-        // * May run into Linux issues that warrant trying out the GL backend.
+        // Native: Everything but DX12
+        // * Wgpu's DX12 impl isn't in a great shape yet and there's now reason to add more variation
+        //   when we can just use Vulkan
+        //   So far, the main reason against it would be that some Windows VMs only provide DX12 drivers,
+        //   observed with Parallels on Apple Silicon. In the future we might want to reconsider
+        //   based on surface/presentation support which may be better with DX12.
+        // * We'd like to exclude GL, but on Linux this can be a very useful fallback for users with
+        //   with old hardware or bad/missing drivers. Wgpu automatically prefers Vulkan over GL when possible.
         //
         // For changing the backend we use standard wgpu env var, i.e. WGPU_BACKEND.
         wgpu::util::backend_bits_from_env()
-            .unwrap_or(wgpu::Backends::VULKAN | wgpu::Backends::METAL)
+            .unwrap_or(wgpu::Backends::VULKAN | wgpu::Backends::METAL | wgpu::Backends::GL)
     } else {
         wgpu::Backends::GL | wgpu::Backends::BROWSER_WEBGPU
     }