Add utility function to detect if SDL is inside a sandbox

Refactor the previous sandbox check in a standalone function that also
includes Snap support.

Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>

src/core/linux/SDL_sandbox.c

@@ -0,0 +1,50 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 2022 Collabora Ltd.
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+
+#include "../../SDL_internal.h"
+#include "SDL_sandbox.h"
+
+#include <unistd.h>
+
+SDL_Sandbox SDL_DetectSandbox(void)
+{
+    if (access("/.flatpak-info", F_OK) == 0) {
+        return SDL_SANDBOX_FLATPAK;
+    }
+
+    /* For Snap, we check multiple variables because they might be set for
+     * unrelated reasons. This is the same thing WebKitGTK does. */
+    if (SDL_getenv("SNAP") != NULL
+        && SDL_getenv("SNAP_NAME") != NULL
+        && SDL_getenv("SNAP_REVISION") != NULL) {
+        return SDL_SANDBOX_SNAP;
+    }
+
+    if (access("/run/host/container-runtime", F_OK) == 0) {
+        return SDL_SANDBOX_UNKNOWN_CONTAINER;
+    }
+
+    return SDL_SANDBOX_NONE;
+}
+
+/* vi: set ts=4 sw=4 expandtab: */