xwayland: enable MIT-SHM shared pixmaps

Allow X11 clients to create shared pixmaps via the MIT-SHM
extension under Xwayland. Tested with a wlroots patch [1].

Also add a few assertions to make sure we have wl_buffers where we
need them.

[1]: https://github.com/swaywm/wlroots/pull/2875

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>

hw/xwayland/xwayland-present.c

@@ -404,6 +404,9 @@ xwl_present_check_flip2(RRCrtcPtr crtc,
     if (!xwl_window)
         return FALSE;
 
+    if (!xwl_glamor_pixmap_get_wl_buffer(pixmap))
+        return FALSE;
+
     if (!xwl_glamor_check_flip(pixmap))
         return FALSE;
 

hw/xwayland/xwayland-screen.c

@@ -52,6 +52,10 @@
 #include "xwayland-present.h"
 #include "xwayland-shm.h"
 
+#ifdef MITSHM
+#include "shmint.h"
+#endif
+
 #include "xdg-output-unstable-v1-client-protocol.h"
 #include "viewporter-client-protocol.h"
 #include "xdg-shell-client-protocol.h"
@@ -662,6 +666,10 @@ xwl_screen_init(ScreenPtr pScreen, int argc, char **argv)
 
     fbPictureInit(pScreen, 0, 0);
 
+#ifdef MITSHM
+    ShmRegisterFbFuncs(pScreen);
+#endif
+
 #ifdef HAVE_XSHMFENCE
     if (!miSyncShmScreenInit(pScreen))
         return FALSE;