SDL: x11: Log a warning if we decide to use XVidMode.

From d5fe9c308a95a3e7784ce0ff7e3067ed0fbee347 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 24 Aug 2021 14:18:47 -0400
Subject: [PATCH] x11: Log a warning if we decide to use XVidMode.

Reference issue #1782.
---
 src/video/x11/SDL_x11modes.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/video/x11/SDL_x11modes.c b/src/video/x11/SDL_x11modes.c
index da520e3115..c0a5b5f27e 100644
--- a/src/video/x11/SDL_x11modes.c
+++ b/src/video/x11/SDL_x11modes.c
@@ -849,6 +849,16 @@ X11_InitModes(_THIS)
     if (_this->num_displays == 0) {
         return SDL_SetError("No available displays");
     }
+
+#if SDL_VIDEO_DRIVER_X11_XVIDMODE
+    if (use_vidmode) {  /* we intend to remove support for XVidMode soon. */
+        SDL_LogWarn(SDL_LOG_CATEGORY_VIDEO, "SDL is using XVidMode to manage your displays!");
+        SDL_LogWarn(SDL_LOG_CATEGORY_VIDEO, "This almost always means either SDL was misbuilt");
+        SDL_LogWarn(SDL_LOG_CATEGORY_VIDEO, "or your X server is insufficient. Please check your setup!");
+        SDL_LogWarn(SDL_LOG_CATEGORY_VIDEO, "Fullscreen and/or multiple displays will not work well.");
+    }
+#endif
+
     return 0;
 }