React to resize events

E.g. when forcing full-screen in Sway this now works!  Without this
change nothing would render after a resize.

zig/sdl/hello_sdl.zig

@@ -48,7 +48,7 @@ pub fn main() !void {
    };
    defer c.SDL_DestroyWindow(screen);
    const surface = c.SDL_GetWindowSurface(screen);
    var surface = c.SDL_GetWindowSurface(screen);
    // assume monospace font
    var glyph_width: c_int = 0;
@@ -70,6 +70,14 @@ pub fn main() !void {
                c.SDL_QUIT => {
                    quit = true;
                },
                c.SDL_WINDOWEVENT => {
                    switch (event.window.event) {
                        c.SDL_WINDOWEVENT_SIZE_CHANGED => {
                            surface = c.SDL_GetWindowSurface(screen);
                        },
                        else => {},
                    }
                },
                c.SDL_KEYDOWN => {
                    switch (event.key.keysym.sym) {
                        c.SDLK_ESCAPE => {