vaxis: add panic handler function

Applications may use this to handle panics gracefully

src/main.zig

@@ -1,7 +1,10 @@
 const std = @import("std");
+const builtin = @import("builtin");
 
 pub const Vaxis = @import("Vaxis.zig");
+
 pub const Loop = @import("Loop.zig").Loop;
+pub const xev = @import("xev.zig");
 
 pub const Queue = @import("queue.zig").Queue;
 pub const Key = @import("Key.zig");
@@ -22,12 +25,29 @@ pub const Winsize = tty.Winsize;
 
 pub const widgets = @import("widgets.zig");
 pub const gwidth = @import("gwidth.zig");
+pub const ctlseqs = @import("ctlseqs.zig");
 
 /// Initialize a Vaxis application.
 pub fn init(alloc: std.mem.Allocator, opts: Vaxis.Options) !Vaxis {
     return Vaxis.init(alloc, opts);
 }
 
+/// Resets terminal state on a panic, then calls the default zig panic handler
+pub fn panic_handler(msg: []const u8, error_return_trace: ?*std.builtin.StackTrace, ret_addr: ?usize) noreturn {
+    if (Tty.global_tty) |gty| {
+        const reset: []const u8 = ctlseqs.csi_u_pop ++
+            ctlseqs.mouse_reset ++
+            ctlseqs.bp_reset ++
+            ctlseqs.rmcup;
+
+        gty.anyWriter().writeAll(reset) catch {};
+
+        gty.deinit();
+    }
+
+    std.builtin.default_panic(msg, error_return_trace, ret_addr);
+}
+
 /// the vaxis logo. In PixelCode
 pub const logo =
     \\▄   ▄  ▄▄▄  ▄   ▄ ▄▄▄  ▄▄▄

src/tty.zig

@@ -35,6 +35,11 @@ pub const PosixTty = struct {
     var handler_mutex: std.Thread.Mutex = .{};
     var handler_idx: usize = 0;
 
+    /// global tty instance, used in case of a panic. Not guaranteed to work if
+    /// for some reason there are multiple TTYs open under a single vaxis
+    /// compilation unit - but this is better than nothing
+    pub var global_tty: ?PosixTty = null;
+
     /// initializes a Tty instance by opening /dev/tty and "making it raw". A
     /// signal handler is installed for SIGWINCH. No callbacks are installed, be
     /// sure to register a callback when initializing the event loop
@@ -56,10 +61,14 @@ pub const PosixTty = struct {
         };
         try posix.sigaction(posix.SIG.WINCH, &act, null);
 
-        return .{
+        const self: PosixTty = .{
             .fd = fd,
             .termios = termios,
         };
+
+        global_tty = self;
+
+        return self;
     }
 
     /// release resources associated with the Tty return it to its original state