core: implement flush and poll

Triggers the following error:
```
Segmentation fault at address 0x520
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:245:24: 0x134f3e8 in writableLength (core-triangle)
            return self.buf.len - self.count;
                       ^
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:136:36: 0x125e015 in ensureUnusedCapacity (core-triangle)
            if (self.writableLength() >= size) return;
                                   ^
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:299:42: 0x125df76 in writeItem (core-triangle)
            try self.ensureUnusedCapacity(1);
                                         ^
/home/josh/dev/open-source/mach/src/Core.zig:379:26: 0x13763b7 in keyboardHandleEnter (core-triangle)
    core.events.writeItem(event) catch {
                         ^
???:?:?: 0x7d27dc80a595 in ??? (libffi.so.8)
Unwind information for `libffi.so.8:0x7d27dc80a595` was not available, trace may be incomplete
```

src/core/linux/Wayland.zig

@@ -178,6 +178,27 @@ pub fn deinit(
 }
 
 pub fn update(wl: *Wayland) !void {
+    while (wl.libwaylandclient.wl_display_flush(wl.display) == -1) {
+        if (std.posix.errno(-1) == std.posix.E.AGAIN) {
+            log.err("flush error", .{});
+            return error.FlushError;
+        }
+        var pollfd = [_]std.posix.pollfd{
+            std.posix.pollfd{
+                .fd = wl.libwaylandclient.wl_display_get_fd(wl.display),
+                .events = std.posix.POLL.OUT,
+                .revents = 0,
+            },
+        };
+        while (try std.posix.poll(&pollfd, 1) == -1) {
+            const errno = std.posix.errno(-1);
+            if (errno == std.posix.E.INTR or errno == std.posix.E.AGAIN) {
+                log.err("poll error", .{});
+                return error.PollError;
+            }
+        }
+    }
+
     _ = wl.libwaylandclient.wl_display_roundtrip(wl.display);
 
     wl.core.input.tick();