Added SDR white point for X11 on Steam Deck

Also disabled HDR on X11 for other devices for now, we don't know that they'll be able to display it properly.

src/video/x11/SDL_x11modes.c

@@ -635,14 +635,32 @@ static bool X11_FillXRandRDisplayInfo(SDL_VideoDevice *_this, Display *dpy, int
     display->internal = displaydata;
 
     if (info) {
-        if (info->max_luminance > 0.0) {
-            /* ITU-R BT.2408-7 (Sept 2023) has the reference PQ white level at 203 nits,
-             * while older Dolby documentation claims a reference level of 100 nits.
-             *
-             * Use 203 nits for now.
-             */
-            display->HDR.HDR_headroom = (float)info->max_luminance / 203.0f;
-            display->HDR.SDR_white_level = 1.0f;
+        /* ITU-R BT.2408-7 (Sept 2023) has the reference PQ white level at 203 nits,
+         * while older Dolby documentation claims a reference level of 100 nits.
+         *
+         * Use 203 nits for now.
+         */
+        float SDR_white_level = 203.0f;
+
+        bool steam_deck = SDL_GetHintBoolean("SteamDeck", false);
+        if (steam_deck) {
+            // The SDR white level is dynamic, but typically is 400 nits
+            // We'll need some other way to get this for external displays
+            SDR_white_level = 400.0f;
+        } else {
+            // Support for HDR on X11 seems spotty, let's disable this for now
+            SDR_white_level = 0.0f;
+        }
+
+        // Allow a hint override for the SDR white level
+        const char *hint = SDL_GetHint("SDL_X11_SDR_WHITE_LEVEL");
+        if (hint && *hint) {
+            SDR_white_level = (float)SDL_atof(hint);
+        }
+
+        if (info->max_luminance > 0.0 && SDR_white_level > 0.0f) {
+            display->HDR.HDR_headroom = (float)info->max_luminance / SDR_white_level;
+            display->HDR.SDR_white_level = SDR_white_level / 80.0f;
         }
 
         SDL_free(info);