Fix X and Y coordinates getting reversed in editorclass::reset()

When editorclass::reset() was resetting the contents of the level
previously, it was mixing up the X and Y bounds. The Y bound was
supposed to be 30*maxheight, and the X bound was supposed to be
40*maxwidth. Instead, it took 30*maxwidth as its Y bound and
40*maxheight as its X bound.

Then, when it actually indexes the contents vector to set each tile to
0, it used 30*maxwidth instead of 40*maxwidth.

The difference between width and height is a bit hard to spot, but one
thing you can do to remember the difference is to remember the fact that
X corresponds with width, and Y corresponds with height. Also, rooms are
40 by 30 tiles, and so X (and therefore width) should correspond with
40, and Y (and therefore height) should correspond with 30.

As a result of mixing up the variables, whenever you played a 20x20 map,
quit the level and then started making a new 20x20 map, the tiles of the
last four rows of the previous map would persist, from y=16 (1-indexed)
all the way to y=20 (1-indexed).

I don't recall anyone ever running into this bug before, which is a bit
strange. But if no one truly has ever ran into this bug before, then I'm
genuinely surprised.

While working on the patch to fix the enemy type room property of each
room not getting reset, and testing the fix, I noticed that for some
reason some contents of the previous level I played in order to test the
enemy type property persisting was ALSO persisting alongside the enemy
type property.

Then I read the code and when I realized that the X and Y bounds were
getting mixed up I groaned. Very loudly.

desktop_version/src/editor.cpp

@@ -319,11 +319,11 @@ void editorclass::reset()
         }
     }
 
-    for (int j = 0; j < 30 * maxwidth; j++)
+    for (int j = 0; j < 30 * maxheight; j++)
     {
-        for (int i = 0; i < 40 * maxheight; i++)
+        for (int i = 0; i < 40 * maxwidth; i++)
         {
-            contents[i+(j*30*maxwidth)]=0;
+            contents[i+(j*40*maxwidth)]=0;
         }
     }