gallium-xlib: query MIT-SHM before using it.

When Mesa is compiled for gallium-xlib using e.g.
./configure --enable-glx=gallium-xlib --disable-dri --disable-gbm
-disable-egl
and is used by an X server (usually remotely via SSH X11 forwarding)
that does not support MIT-SHM such as XMing or MobaXterm, OpenGL
clients report error messages such as
Xlib:  extension "MIT-SHM" missing on display "localhost:11.0".
ad infinitum.

The reason is that the code in src/gallium/winsys/sw/xlib uses
MIT-SHM without checking for its existence, unlike the code
in src/glx/drisw_glx.c and src/mesa/drivers/x11/xm_api.c.
I copied the same check using XQueryExtension, and tested with
glxgears on MobaXterm.

This issue was reported before here:
https://lists.freedesktop.org/archives/mesa-users/2016-July/001183.html

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: <mesa-stable@lists.freedesktop.org>

src/gallium/winsys/sw/xlib/xlib_sw_winsys.c

@@ -396,6 +396,7 @@ xlib_displaytarget_create(struct sw_winsys *winsys,
 {
    struct xlib_displaytarget *xlib_dt;
    unsigned nblocksy, size;
+   int ignore;
 
    xlib_dt = CALLOC_STRUCT(xlib_displaytarget);
    if (!xlib_dt)
@@ -410,7 +411,8 @@ xlib_displaytarget_create(struct sw_winsys *winsys,
    xlib_dt->stride = align(util_format_get_stride(format, width), alignment);
    size = xlib_dt->stride * nblocksy;
 
-   if (!debug_get_option_xlib_no_shm()) {
+   if (!debug_get_option_xlib_no_shm() &&
+       XQueryExtension(xlib_dt->display, "MIT-SHM", &ignore, &ignore, &ignore)) {
       xlib_dt->data = alloc_shm(xlib_dt, size);
       if (xlib_dt->data) {
          xlib_dt->shm = True;