Warn on MIT-SHM not working on Linux X11 (#6856)

* Clarify video capture method

* fix improper level of pointer usage of xcb_generic_error_t

* add ffi of xcb_shm_query_version

* throw a warn about MIT-SHM not working

* add missing #[cfg]

* checks SHM validity on the fly, rather than cache on creation

---------

Co-authored-by: root <root@localhost>
Co-authored-by: rustdesk-fork <rustdesk@fork.com>

libs/scrap/src/x11/server.rs

@@ -87,6 +87,24 @@ impl Server {
     pub fn setup(&self) -> *const xcb_setup_t {
         self.setup
     }
+    pub fn get_shm_status(&self) -> Result<(), Error> {
+        unsafe { check_x11_shm_available(self.raw) }
+    }
+}
+
+unsafe fn check_x11_shm_available(c: *mut xcb_connection_t) -> Result<(), Error> {
+    let cookie = xcb_shm_query_version(c);
+    let mut e: *mut xcb_generic_error_t = std::ptr::null_mut();
+    let reply = xcb_shm_query_version_reply(c, cookie, &mut e as _);
+    if reply.is_null() {
+        // TODO: Should seperate SHM disabled from SHM not supported?
+        return Err(Error::UnsupportedExtension);
+    } else if e.is_null() {
+        return Ok(());
+    } else {
+        // TODO: Does "This request does never generate any errors" in manual means `e` is never set, so we would never reach here?
+        return Err(Error::Generic);
+    }
 }
 
 impl Drop for Server {