Add screenshot of web UI to README.md, add back download button, misc.

Added assert(0) to the end of main() in hitomezashi_web.c indicating that it
is unreachable.

4 files changed, 7 insertions, 2 deletions

diff --git a/README.md b/README.md
index a780310..abc4526 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,8 @@ To build, first run `make clean` if you previously built to native code, then (w
 
 You will need to serve, from a web server: `hitomezashi_web.html`, `hitomezashi_web.js`, `hitomezashi_web.wasm` in order for it to work; the `file://` protocol may not be sufficient.
 
+<img src="https://github.com/untir-l/hitomezashi/blob/main/screenshot_web.png?raw=true" width="500">
+
 ## Technical and copyright information
 
 Written in C11 with SDL2. Code style: `make format-code` will run `clang-format` with the correct parameters.
diff --git a/screenshot_web.png b/screenshot_web.png
new file mode 100644
index 0000000..4a6c0ba
--- /dev/null
+++ b/screenshot_web.png
diff --git a/web/hitomezashi_web.c b/web/hitomezashi_web.c
index 994eaa5..7e756ff 100644
--- a/web/hitomezashi_web.c
+++ b/web/hitomezashi_web.c
@@ -3,6 +3,7 @@
 #include "emscripten.h"
 #include "hitomezashi.h"
 #include "hitomezashi_utils.h"
+#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
@@ -21,7 +22,7 @@ int EMSCRIPTEN_KEEPALIVE main(void) {
 
   emscripten_set_main_loop(main_loop, -1, true);
 
-  return EXIT_SUCCESS;
+  assert(0); // This should never be reached
 }
 
 void EMSCRIPTEN_KEEPALIVE main_loop(void) {
diff --git a/web/hitomezashi_web_shell.html b/web/hitomezashi_web_shell.html
index e0e23d4..8e53afe 100644
--- a/web/hitomezashi_web_shell.html
+++ b/web/hitomezashi_web_shell.html
@@ -58,8 +58,10 @@
     <p><label>foreground colour: <input name="fg_colour" type="color"></label></p>
     <p><label>background colour: <input name="bg_colour" type="color"></label></p>
   </form>
-
+  <br>
   <canvas class="emscripten" id="canvas" oncontextmenu="event.preventDefault()" tabindex=-1></canvas>
+  <br>
+  <button id="downloadCanvas">download as image</button>
   <script type='text/javascript'>
     var Module = {
       canvas: (function () {