Point at software OpenGL when OpenGL fails to initialize on X11

(cherry picked from commit 8fbdcb6fea6d075bb934e99711bf0cb44b66cfc0)

platform/osx/os_osx.mm

@@ -1763,7 +1763,7 @@ Error OS_OSX::initialize(const VideoMode &p_desired, int p_video_driver, int p_a
 
 	if (gl_initialization_error) {
 		OS::get_singleton()->alert("Your video card driver does not support any of the supported OpenGL versions.\n"
-								   "Please update your drivers or if you have a very old or integrated GPU upgrade it.",
+								   "Please update your drivers or if you have a very old or integrated GPU, upgrade it.",
 				"Unable to initialize Video driver");
 		return ERR_UNAVAILABLE;
 	}

platform/uwp/os_uwp.cpp

@@ -240,7 +240,7 @@ Error OS_UWP::initialize(const VideoMode &p_desired, int p_video_driver, int p_a
 
 	if (gl_initialization_error) {
 		OS::get_singleton()->alert("Your video card driver does not support any of the supported OpenGL versions.\n"
-								   "Please update your drivers or if you have a very old or integrated GPU upgrade it.",
+								   "Please update your drivers or if you have a very old or integrated GPU, upgrade it.",
 				"Unable to initialize Video driver");
 		return ERR_UNAVAILABLE;
 	}

platform/windows/os_windows.cpp

@@ -1644,7 +1644,7 @@ Error OS_Windows::initialize(const VideoMode &p_desired, int p_video_driver, int
 
 	if (gl_initialization_error) {
 		OS::get_singleton()->alert("Your video card driver does not support any of the supported OpenGL versions.\n"
-								   "Please update your drivers or if you have a very old or integrated GPU upgrade it.",
+								   "Please update your drivers or if you have a very old or integrated GPU, upgrade it.",
 				"Unable to initialize Video driver");
 		return ERR_UNAVAILABLE;
 	}

platform/x11/os_x11.cpp

@@ -357,7 +357,9 @@ Error OS_X11::initialize(const VideoMode &p_desired, int p_video_driver, int p_a
 
 	if (gl_initialization_error) {
 		OS::get_singleton()->alert("Your video card driver does not support any of the supported OpenGL versions.\n"
-								   "Please update your drivers or if you have a very old or integrated GPU upgrade it.",
+								   "Please update your drivers or if you have a very old or integrated GPU, upgrade it.\n"
+								   "Alternatively, you can force software rendering by running Godot with the `LIBGL_ALWAYS_SOFTWARE=1`\n"
+								   "environment variable set, but this will be very slow.",
 				"Unable to initialize Video driver");
 		return ERR_UNAVAILABLE;
 	}