Draw input hints to a separate string

This makes it possible to actually run things after deleting
characters...

zig/sdl/hello_sdl.zig

    const renderer = c.SDL_CreateRenderer(screen, -1, c.SDL_RENDERER_ACCELERATED);
    var msg = "                                                                                                    ".*;
    var msg_overlay = "                                                                                                    ".*;
    var pos: usize = 0;
    var max_chars = std.math.min(@divTrunc(@intCast(usize, window_width), @intCast(usize, glyph_width)), msg.len);
                    if (ctrlPressed) {
                        switch (event.key.keysym.sym) {
                            c.SDLK_a => {
                                if (msg_overlay[pos] == '_') {
                                    msg_overlay[pos] = ' ';
                                }
                                pos = 0;
                                msg[pos] = '_';
                                msg_overlay[pos] = '_';
                            },
                            c.SDLK_k => {
                                var i: usize = 0;
                                quit = true;
                            },
                            c.SDLK_BACKSPACE => {
                                if (msg_overlay[pos] == '_') {
                                    msg_overlay[pos] = ' ';
                                }
                                if (pos > 0) {
                                    msg[pos - 1] = ' ';
                                }
                                pos = if (pos == 0) max_chars - 1 else (pos - 1) % (max_chars - 1);
                                msg[pos] = '_';
                                msg_overlay[pos] = '_';
                                changed = true;
                            },
                            c.SDLK_RETURN => {
                    if (!ctrlPressed and event.text.text.len > 0) {
                        c.SDL_Log("input: '%s' at %d", event.text.text, pos);
                        msg[pos] = event.text.text[0];
                        msg_overlay[pos] = ' ';
                        pos = (pos + 1) % (max_chars - 1);
                        changed = true;
        _ = c.SDL_RenderClear(renderer);
        // thanks to https://stackoverflow.com/questions/22886500/how-to-render-text-in-sdl2 for some actually useful code here
        const white: c.SDL_Color = c.SDL_Color{ .r = 255, .g = 255, .b = 255, .a = 0 };
        const white: c.SDL_Color = c.SDL_Color{ .r = 255, .g = 255, .b = 255, .a = 255 };
        const gray: c.SDL_Color = c.SDL_Color{ .r = 150, .g = 150, .b = 150, .a = 255 };
        const black: c.SDL_Color = c.SDL_Color{ .r = 0, .g = 0, .b = 0, .a = 100 };
        // Shaded vs Solid gives a nicer output, with solid the output
        // was squiggly and not aligned with a baseline.
        const text = c.TTF_RenderUTF8_Shaded(font, &msg, white, black);
        const texture = c.SDL_CreateTextureFromSurface(renderer, text);
        c.SDL_FreeSurface(text);
        _ = c.SDL_RenderCopy(renderer, texture, null, &c.SDL_Rect{ .x = 0, .y = 0, .w = @intCast(c_int, msg.len) * glyph_width, .h = glyph_height });
        const black: c.SDL_Color = c.SDL_Color{ .r = 0, .g = 0, .b = 0, .a = 255 };
        {
            // Shaded vs Solid gives a nicer output, with solid the output
            // was squiggly and not aligned with a baseline.
            const text = c.TTF_RenderUTF8_Shaded(font, &msg, white, black);
            const texture = c.SDL_CreateTextureFromSurface(renderer, text);
            c.SDL_FreeSurface(text);
            _ = c.SDL_RenderCopy(renderer, texture, null, &c.SDL_Rect{ .x = 0, .y = 0, .w = @intCast(c_int, msg.len) * glyph_width, .h = glyph_height });
        }
        {
            const text = c.TTF_RenderUTF8_Shaded(font, &msg_overlay, white, c.SDL_Color{ .r = 0, .g = 0, .b = 0, .a = 0 });
            const texture = c.SDL_CreateTextureFromSurface(renderer, text);
            if (c.SDL_SetTextureBlendMode(texture, c.SDL_BLENDMODE_ADD) != 0) {
                c.SDL_Log("Unable set texture blend mode: %s", c.SDL_GetError());
            }
            c.SDL_FreeSurface(text);
            _ = c.SDL_RenderCopy(renderer, texture, null, &c.SDL_Rect{ .x = 0, .y = 0, .w = @intCast(c_int, msg.len) * glyph_width, .h = glyph_height });
        }
        var i: c_int = 1;
        for (commands) |*command| {