js: keep colour strings in JavaScript rather than in C

The drawing routines in JavaScript used to take pointers to a C string
containing a CSS colour name.  That meant that JavaScript had to create
a new JavaScript string on ever call to a drawing function, which seemed
ugly.

So now we instead pass colour numbers all the way down into JavaScript
and keep an array of JavaScript strings there that can be re-used.  The
conversion from RGB triples to strings is still done in C, though.

This doesn't seem to have fixed either of the bugs I hoped it would, but
it does measurably improve drawing performance so I think it's worth
doing.

emccpre.js

@@ -41,6 +41,10 @@ var ctx;
 // by js_canvas_end_draw.
 var update_xmin, update_xmax, update_ymin, update_ymax;
 
+// Colour strings to use when drawing, to save converting them from C
+// every time.
+var colours = [];
+
 // Module object for Emscripten. We fill in these parameters to ensure
 // that when main() returns nothing will get cleaned up so we remain
 // able to call the puzzle's various callbacks.