Add feature to use gamescope for clients

- The launcher is responsible for subreaping in behalf of clients, however, we're not able to fullfill that responsibility if gamescope is used which will require clients to handle killing the zombie wine processes if the game had exited improperly (e.g., closing the game window). Currently, all GUI launchers that do not implement a sophisticated subreaping system will need to do this. However, by executing gamescope via ULWGL, we can ensure all zombies are killed via systemd when the user exits improperly

- This will be an optional feature for GUI clients by requiring to set ULWGL_GAMESCOPE=1. To pass gamescope options to the launcher, clients must write a JSON file in ~/.local/share/ULWGL/state/<ulwgl-id>.json with the gamescope options. For example:

[
 {
   'name': 'gamescope',
   'options': ['-F', 'fsr']
 }
]

ULWGL/ulwgl_plugins.py

@@ -6,6 +6,7 @@
 from shutil import which
 from ulwgl_log import log
 from ulwgl_consts import TOMLDocument, ULWGL_LOCAL
+from json import load
 
 
 def set_env_toml(
@@ -110,6 +111,24 @@ def _check_env_toml(env: Dict[str, str], toml: TOMLDocument) -> Dict[str, Any]:
                 f"Please specify a value or remove the entry:\n{key} = {val}"
             )
             raise ValueError(err)
+        if key == "gamescope" and not isinstance(val, bool):
+            err: str = (
+                f"Value is not a boolean for '{key}' in TOML.\n"
+                f"Please specify a value or remove the entry:\n{key} = {val}"
+            )
+            raise ValueError(err)
+
+    if (
+        toml.get("plugins")
+        and toml.get("plugins").get("gamescope")
+        and toml.get("plugins").get("gamescope").get("options")
+        and not isinstance(toml.get("plugins").get("gamescope").get("options"), list)
+    ):
+        err: str = (
+            f"Value is not an array for '{key}' in TOML.\n"
+            f"Please specify a value or remove the entry:\n{key} = {val}"
+        )
+        raise ValueError(err)
 
     return toml
 
@@ -235,3 +254,41 @@ def enable_systemd(env: Dict[str, str], command: List[str]) -> List[str]:
     )
 
     return command
+
+
+def enable_gamescope(
+    env: Dict[str, str], command: List[str], opts: List[str] = None
+) -> List[str]:
+    """Enable the gamescope microcompositor."""
+    bin: str = which("gamescope")
+    id: str = env["ULWGL_ID"]
+    json: Dict[str, Any] = None
+
+    if not id.startswith("ulwgl-"):
+        id = f"ulwgl-{id}"  # We use an underscore for the file
+
+    if not bin or not ULWGL_LOCAL.joinpath("state", f"{id}.json"):
+        return command
+
+    # Config
+    if opts:
+        command.extend([bin, *opts, "--"])
+        return command
+
+    # CLI
+    with ULWGL_LOCAL.joinpath("state", f"{id}.json").open(mode="r") as file:
+        json = load(file)
+
+    # Pass user options
+    # NOTE: We do not validate them. We trust the client
+    for item in json:
+        if item.get("name") == "gamescope":
+            opts = item.get("options")
+
+    if opts:
+        command.extend([bin, *opts, "--"])
+        return command
+
+    command.extend([bin, "--"])
+
+    return command

ULWGL/ulwgl_run.py

@@ -20,6 +20,8 @@
     set_env_toml,
     enable_reaper,
     enable_systemd,
+    enable_gamescope,
+)
 from ulwgl_consts import (
     PROTON_VERBS,
     DEBUG_FORMAT,
@@ -270,6 +272,22 @@ def build_command(
         log.debug("Using reaper as subreaper")
         enable_reaper(env, command, local)
 
+    # Gamescope
+    if env.get("ULWGL_GAMESCOPE") == "1":
+        log.debug("Using gamescope")
+        enable_gamescope(env, command)
+    elif (
+        config
+        and config.get("ulwgl").get("gamescope")
+        and config.get("plugins")
+        and config.get("plugins").get("gamescope")
+        and config.get("plugins").get("gamescope").get("options")
+    ):
+        log.debug("Using gamescope")
+        enable_gamescope(
+            env, command, config.get("plugins").get("gamescope").get("options")
+        )
+
     command.extend([local.joinpath("ULWGL").as_posix(), "--verb", verb, "--"])
     command.extend(
         [
@@ -307,6 +325,7 @@ def main() -> int:  # noqa: D103
         "STORE": "",
         "PROTON_VERB": "",
         "ULWGL_ID": "",
+        "ULWGL_GAMESCOPE": "",
         "ULWGL_SYSTEMD": "",
     }
     command: List[str] = []