Set canvas background to black...

..to avoid blending issues with white background

templates/web_shell/shell.html

@@ -57,7 +57,7 @@
       div.emscripten_border { border: 1px solid black; }
       
       /* the canvas *must not* have any border or padding, or mouse coords will be wrong */
-      canvas.emscripten { border: 0px none; }
+      canvas.emscripten { border: 0px none; background: black; }
       
       #emscripten_logo {
         display: inline-block;