vaxis: fix window initialization

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>

examples/main.zig

@@ -33,6 +33,9 @@ pub fn main() !void {
             },
             else => {},
         }
+
+        const win = vx.window();
+        _ = win;
     }
 }
 

src/vaxis.zig

@@ -93,9 +93,8 @@ pub fn Vaxis(comptime T: type) type {
             return Window{
                 .x_off = 0,
                 .y_off = 0,
-                .w = .{.expand},
+                .width = self.screen.width,
-                .h = .{.expand},
+                .height = self.screen.height,
-                .parent = null,
                 .screen = &self.screen,
             };
         }