fdo: Factor out switcheroos for the different Wayland shell protocols

[aperezdc]

Currently in the FDO Wayland platform plug-in we have code that checks which shell protocol is being used, every time that is needed. Like this:

if (wl_data.xdg_shell) {
    // do something when we use the xdg_wm_base protocol
} else if (wl_data.fshell) {
    // do something else when using the zwp_fullscreen_shell_v1 protocol
}  else if (wl_data.shell) {
    // and something different for the wl_shell_protocol
} else {
    g_assert_not_reached();
}

It would be better to avoid duplicating this logic everywhere, and instead:

• Have a list of functions for each operation used from the different protocols (initialize surface, enter fullscreen, exit fullscreen, deinitialize surface, set title, etc.)
  • This can a struct shell_operations with a function pointer for each operation.
  • For each shell protocol, have a static const instance of the struct with the function pointers set to the corresponding functions.
• Have the notion of “current shell protocol being used”.
  • This can be a pointer to one of the struct shell_operations, its value determined at runtime depending on which protocol(s) the Wayland compositor supports.
• Instead of Wayland protocol functions directly, do the calls through the struct with callbacks pointed to as “current shell protocol”.