Enable use of GL_EXT_multisampled_render_to_texture extension for OpenGL backend

[opstic]

Connections
Depends on grovesNL/glow#352.

Description
Enables using the GL_EXT_multisampled_render_to_texture extension when available. It enables direct multisampled rendering into a single sampled texture without needing an extra resolve operation. Saving bandwidth for mobile devices.

Testing
Tested with a mobile app and looking at graphics traces/output to make sure the resolve operation is skipped and multisampling is still applied correctly.

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[Wumpf]

Thanks! Looks all good to me, but we'll ofc have to wait for the glow update here. Also, it's still missing a changelog entry.

@grovesNL could you please have a look?