Unify all queries to map size to map.getwidth and map.getheight

It's becoming pretty clear that the size of the map is important enough
to be queried a lot, but each time it's something like `map.custommode ?
map.customwidth : 20` and `map.custommode ? map.customheight : 20` which
is not ideal because of copy-pasting.

Furthermore, even `map.customwidth` and `map.customheight` are just
duplicates of `cl.mapwidth` and `cl.mapheight`, which are only set in
`customlevelclass::generatecustomminimap`. This is a bit annoying if you
want to, say, add checks that depend on the width and height of the
custom map in `mapclass::initcustommapdata`, but `map.customwidth` and
`map.customheight` are out of date because `generatecustomminimap`
hasn't been called yet. And doing the ternary there requires a `#ifndef
NO_CUSTOM_LEVELS` to reference `cl.mapwidth` and `cl.mapheight` which is
just awful.

So I'm axing `map.customwidth` and `map.customheight`, and I'm axing all
the ternaries that are duplicating the source of truth in
`MapRenderData`. Instead, there will just be one function to call for
the width and height, `mapclass::getwidth` and `mapclass::getheight`,
and everyone can simply call those without needing to do ternaries or
duplication.

desktop_version/src/Map.h

@@ -23,6 +23,10 @@ class mapclass
 public:
     mapclass(void);
 
+    int getwidth(void);
+
+    int getheight(void);
+
     int intpol(int a, int b, float c);
 
     void setteleporter(int x, int y);
@@ -129,7 +133,6 @@ public:
     //Variables for playing custom levels
     bool custommode;
     bool custommodeforreal;
-    int customwidth, customheight;
     int custommmxoff, custommmyoff, custommmxsize, custommmysize;
     int customzoom;
     bool customshowmm;