Fix hxsl checker bug where const can't have the same name with import with another shader.

The fix consist in reorder the check of double declaration with the skip of params.

hxsl/Checker.hx

@@ -799,12 +799,13 @@ class Checker {
 					checkConst(e);
 					einit = e;
 				}
-				if( v.type == null ) error("Type required for variable declaration", e.pos);
-				if( vars.exists(v.name) ) error("Duplicate var decl '" + v.name + "'", e.pos);
-				var v = makeVar(v, e.pos);
+				if( v.type == null ) error("Type required for variable declaration", e.pos);				
 				if( isImport && v.kind == Param )
 					continue;
 
+				if( vars.exists(v.name) ) error("Duplicate var decl '" + v.name + "'", e.pos);
+				var v = makeVar(v, e.pos);
+
 				switch( v.type ) {
 				case TSampler(T3D, true), TRWTexture(T3D, true, _), TRWTexture(_,_,3):
 					error("Unsupported texture type", e.pos);