This PR adds two utility functions, designed to be used by xdg-desktop-portal-luminous:

• get_available_frame_formats: returns a vector of valid formats for a given WlOutput. The original idea was to pick a random WlOutput from the connection, performing a 1-pixel screenshot to obtain the valid formats using wlr-screencopy protocol, and saving them in a LazyCell. I preferred to capture the entire screen from a given WlOutput passed as parameter instead, to return a vector of FrameFormat which contain additional information such as the size and the stride of the buffer. This makes the function more general purpose, allowing it to be used in multiple contexts. For example, right now only 4-bytes pixel formats are supported; if other pixel formats need to be supported in the future, this information could be useful to properly set PipeWire streams' parameters.
• capture_output_frame_shm_fd_with_format: same as capture_output_frame_shm_fd, but enforces a format to perform the screen capture. Needed since the PipeWire server communicates a preferred video format among the available ones, that must be enforced when capturing video frames.

Additionally, some refactoring was done to avoid code repetitions and pub visibility modifier was removed from capture_output_frame_get_state_shm, as it was added in an attempt to tackle the issues that get_available_frame_formats solves.