add evfb (evdev + fbdev) backend

Outside of a terminal emulator or web browser, this typing experience
can be useful outside the DE, such as in the TTY or when entering a
password for full-disk encryption. Add a target that uses the bare
hardware to allow the keyboard to be deployed in more flexible
environments.

src/ufkbd_evfb.rs

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ * Copyright (c) 2025, Richard Acayan. All rights reserved.
+ */
+
+mod core;
+mod evdev;
+mod fbdev;
+mod linux;
+
+use crate::core::Button;
+use crate::core::Configuration;
+use crate::core::Display;
+use crate::core::Graphics;
+use crate::core::Layout;
+use crate::evdev::Touchscreen;
+use crate::evdev::VirtualKeyboard;
+use crate::fbdev::Framebuffer;
+use crate::linux::Teletype;
+use polling::Event;
+use polling::Events;
+use polling::Poller;
+use signal_hook::consts::signal;
+use signal_hook::iterator::Signals;
+use std::sync::Arc;
+use std::sync::Mutex;
+use std::time::Instant;
+
+fn main()
+{
+    let cfg = Configuration::load().unwrap();
+    let disp = Framebuffer::new(&cfg).unwrap();
+
+    let gfx = Graphics::new(disp, &cfg);
+    let gfx = Mutex::new(gfx);
+    let gfx = Arc::new(gfx);
+
+    let tty = Teletype::new(&cfg).unwrap();
+    let kbd = VirtualKeyboard::new(tty).unwrap();
+    let layout = Layout::load(&cfg).unwrap();
+    let mut btn = Button::new(&cfg, layout, kbd, gfx.clone());
+    btn.set_text_supported(true);
+
+    {
+        let mut gfx = gfx.lock().unwrap();
+        let (width, height) = gfx.display().size();
+        gfx.resize(btn.layout(), btn.mod_state(), width, height);
+    }
+
+    let mut touchscreen = Touchscreen::new(btn, &cfg).unwrap();
+
+    let mut events = Events::new();
+    let poller = Poller::new().unwrap();
+    let ts_evt = Event::readable(0);
+
+    let mut sigs = Signals::new([
+        signal::SIGHUP,
+        signal::SIGINT,
+        signal::SIGPIPE,
+        signal::SIGTERM,
+    ]).unwrap();
+
+    while sigs.pending().next().is_none() {
+        let timer = touchscreen.button().next_time().map(|t| t - Instant::now());
+
+        unsafe {
+            poller.add(&touchscreen, ts_evt).unwrap();
+        }
+
+        events.clear();
+        poller.wait(&mut events, timer).unwrap();
+        poller.delete(&touchscreen).unwrap();
+
+        if !events.is_empty() {
+            touchscreen.handle_events();
+        }
+
+        touchscreen.dispatch_timers();
+    }
+}