Vaxis: make render independent of resize

src/InternalScreen.zig

···
     }
 };
 
-arena: *std.heap.ArenaAllocator = undefined,
+arena: *std.heap.ArenaAllocator,
 width: u16 = 0,
 height: u16 = 0,
 
-buf: []InternalCell = undefined,
+buf: []InternalCell,
 
 cursor_row: u16 = 0,
 cursor_col: u16 = 0,

src/Vaxis.zig

···
     return .{
         .opts = opts,
         .screen = .{},
-        .screen_last = try .init(alloc, 80, 24),
+        .screen_last = try .init(alloc, 0, 0),
         .unicode = try Unicode.init(alloc),
     };
 }