Bug fix (#15)

* increase height of preview canvas image in demo

* small sized curves get cutoff (e.g. 3x3) because the window scaling
  factor is only 1.1 and the margins are significant at small scales.
  Increasing the height factor to 2 (from 1.1) alleviates the problem
  while minimizing the risk of the canvas spilling over horizontally.

* resolves #11

* vestigial test code that caused `gilbert_d2xy(&x, &y, 7861, 490, 490)` to incorrectly return
  -1 (there was a `max_iter` check to return after some hard coded value)
* added test to make sure at least that condition is tested for
* `gilbert.c` had buffer overrun setting last byte of `buf`, correctly sets `buf[1023]='\0'`
  instead of `buf[1024]`

* bug fix

* exported functions got switched (js), fixed

ports/gilbert.js

@@ -585,11 +585,11 @@ function gilbert_d2xyz_r(dst_idx, cur_idx, p, a, b, c) {
 
 if (typeof module !== "undefined") {
 
-  module.exports["d2xy"] = gilbert_xy2d;
-  module.exports["xy2d"] = gilbert_d2xy;
+  module.exports["d2xy"] = gilbert_d2xy;
+  module.exports["xy2d"] = gilbert_xy2d;
 
-  module.exports["d2xyz"] = gilbert_xyz2d;
-  module.exports["xyz2d"] = gilbert_d2xyz;
+  module.exports["d2xyz"] = gilbert_d2xyz;
+  module.exports["xyz2d"] = gilbert_xyz2d;
 
   module.exports["main"] = _main;