plug-ins: Load valid CERN mappings

For CERN-format image maps, GIMP uses "rect", "poly", and "circ"
to represent the relevant shapes. However, "rectangle", "polygon",
and "circle" are also valid. This patch adds those rules to the Lex file
so that we can load mappings created outside of GIMP (like in
Libre Office).
They are added above our comment rules, as other programs are
unlikely to use our comment style.

plug-ins/imagemap/imap_cern.l

@@ -45,6 +45,10 @@ WS	[ \t\n]+
 
 %%
 
+RECTANGLE           return RECTANGLE;
+CIRCLE              return CIRCLE;
+POLYGON             return POLYGON;
+
 RECT{WS}\(4096,4096\){WS}\(4096,4096\).*#\$AUTHOR: {
 				   BEGIN(comment);
 				   return AUTHOR;
@@ -66,6 +70,8 @@ RECT{WS}\(4096,4096\){WS}\(4096,4096\)   {
    				   return COMMENT;
 				}
 
+
+
 RECT				return RECTANGLE;
 
 CIRC				return CIRCLE;