Add set_log_level on ffmpeg

[kipcole9]

Per issue #45, here is a proposed PR to allow setting the ffmpeg log level with either Xav.Reader.set_log_level/1. Compiled and tested on my MacBook Pro with ffmpeg@7.

Summary

Adds a thin Elixir wrapper around FFmpeg's av_log_set_level(int). Also adds an application-env-driven default so applications can configure the level once in config/runtime.exs instead of calling the setter from their own Application.start/2. You may want to remove the second part.

Background

libswscale prints informational lines to stderr at FFmpeg's default log level (AV_LOG_INFO) whenever it falls back to a generic C conversion path instead of a SIMD-optimised one, for example:

[swscaler @ 0x1490a0000] No accelerated colorspace conversion found from yuv420p to rgb24.

Today there is no way to silence the notices from Elixir code because Xav does not expose av_log_set_level/1. Users have to either redirect stderr wholesale (which also hides real errors) or rebuild FFmpeg with the missing SIMD paths.

Design

• Wrapper lives on Xav.Reader to keep it next to the most obvious caller, even though av_log_set_level/3 is process-global and affects every libav* call made from the VM (reader, decoder, encoder).

• Accepts atoms: (:quiet | :panic | :fatal | :error | :warning | :info | :verbose | :debug | :trace) rather than FFmpeg's integer constants, so callers don't need to know the magic numbers. The atom → integer mapping is the Elixir side; the NIF itself just takes an int.

• Application-env default: A new Xav.Application module reads Application.get_env(:xav, :log_level) once at application start and applies it. Users who want quiet decoding globally just add:

  # config/runtime.exs
  config :xav, log_level: :error

• No behaviour change by default. If :log_level is unset, Xav.Application.start/2 leaves FFmpeg's default untouched (AV_LOG_INFO), so existing users see identical output.

Questions

• Should the wrapper live on Xav instead of Xav.Reader?
• Should the default log level come from an env var (e.g. XAV_LOG_LEVEL) as well as the application env, to match what FFmpeg users might already expect? Easy to add — maybe_set_log_level/0 would check both in order.
• Is use Application + a no-children supervisor acceptable as a minimal app callback - its only there to read the config and set log level.