wayland: Allocate the cursor shared memory buffer entirely in memory, if possible.

Use memfd_create() to allocate the temporary SHM backing file in memory, and set the size with posix_fallocate(), which will return an error on insufficient space vs ftruncate(), which will silently succeed and allow a SIGBUS error to occur if the unbacked memory is accessed.

Additionally, make the legacy path more robust by unlinking the temp file, so it won't persist after close, and unmapping the shared memory buffer.

(cherry picked from commit 9bdb992925)
(cherry picked from commit d3c89bb479)

include/SDL_config.h.cmake

@@ -190,6 +190,8 @@
 #cmakedefine HAVE_FOPEN64 1
 #cmakedefine HAVE_FSEEKO 1
 #cmakedefine HAVE_FSEEKO64 1
+#cmakedefine HAVE_MEMFD_CREATE 1
+#cmakedefine HAVE_POSIX_FALLOCATE 1
 #cmakedefine HAVE_SIGACTION 1
 #cmakedefine HAVE_SA_SIGACTION 1
 #cmakedefine HAVE_SETJMP 1