Add support for XDG_SESSION_TYPE env variable

Related to #2439

README.md

@@ -164,6 +164,7 @@ information on what to include when reporting a bug.
  - Added `GLFW_BUILD_X11` CMake option for enabling X11 support (#1958)
  - Added `GLFW_LIBRARY_TYPE` CMake variable for overriding the library type
    (#279,#1307,#1497,#1574,#1928)
+ - Added support for `XDG_SESSION_TYPE` environment variable
  - Added `GLFW_PKG_CONFIG_REQUIRES_PRIVATE` and `GLFW_PKG_CONFIG_LIBS_PRIVATE` CMake
    variables exposing pkg-config dependencies (#1307)
  - Made joystick subsystem initialize at first use (#1284,#1646)

include/GLFW/glfw3.h

@@ -2199,6 +2199,13 @@ typedef struct GLFWallocator
  *  application locale according to the current environment if that category is
  *  still "C".  This is because the "C" locale breaks Unicode text input.
  *
+ *  @remark __Wayland, X11:__ If the library was compiled with support for both
+ *  Wayland and X11, and the @ref GLFW_PLATFORM init hint is set to
+ *  `GLFW_ANY_PLATFORM`, the `XDG_SESSION_TYPE` environment variable affects
+ *  which platform is picked.  If the environment variable is not set, or is set
+ *  to something other than `wayland` or `x11`, the regular detection mechanism
+ *  will be used instead.
+ *
  *  @thread_safety This function must only be called from the main thread.
  *
  *  @sa @ref intro_init

src/platform.c

@@ -27,6 +27,9 @@
 
 #include "internal.h"
 
+#include <string.h>
+#include <stdlib.h>
+
 // These construct a string literal from individual numeric constants
 #define _GLFW_CONCAT_VERSION(m, n, r) #m "." #n "." #r
 #define _GLFW_MAKE_VERSION(m, n, r) _GLFW_CONCAT_VERSION(m, n, r)
@@ -80,6 +83,22 @@ GLFWbool _glfwSelectPlatform(int desiredID, _GLFWplatform* platform)
         return GLFW_FALSE;
     }
 
+#if defined(_GLFW_WAYLAND) && defined(_GLFW_X11)
+    if (desiredID == GLFW_ANY_PLATFORM)
+    {
+        const char* const session = getenv("XDG_SESSION_TYPE");
+        if (session)
+        {
+            // Only follow XDG_SESSION_TYPE if it is set correctly and the
+            // environment looks plausble; otherwise fall back to detection
+            if (strcmp(session, "wayland") == 0 && getenv("WAYLAND_DISPLAY"))
+                desiredID = GLFW_PLATFORM_WAYLAND;
+            else if (strcmp(session, "x11") == 0 && getenv("DISPLAY"))
+                desiredID = GLFW_PLATFORM_X11;
+        }
+    }
+#endif
+
     if (desiredID == GLFW_ANY_PLATFORM)
     {
         // If there is exactly one platform available for auto-selection, let it emit the