Fix output args for render_to_texture example

[ratmice]

Connections
Link to the issues addressed by this PR, or dependent PRs in other repositories

Description
The render_to_texture instructions specified render-to-texture as a --bin render-to-texture rather than wgpu-examples.
Since the example (e.g. render_to_texture) is now specified as the first argument, the output argument should now be the second one.

This also adds the default output to .gitignore.

Currently it will try to create a file named render_to_texture which when running under examples/src already exists and is a directory, which fails.

Testing
cargo run --bin wgpu-examples -- render_to_texture "test.png"

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[Wumpf]

nice catch(es). Thanks for cleaning this up!