bugfix: labels missing on some ATI cards

some ATI cards require the first shader attribute at position 0 to
be bound.  The problem was that the attr_pickcolor attribute was
automatically bound to position 0, and was not always being used, which
caused the ATI card to fail (not exactly sure why, but binding position 0
fixes the problem)

binding also did not always work in the open-source version, since ATI
assigns the attribute positions in alphabetical order, changing the
name of the optional attr_pickcolor attribute seems to fix the labels
on these cards

data/shaders/label.vs

@@ -1,8 +1,11 @@
+
 attribute vec4 attr_worldpos;
 attribute vec3 attr_screenoffset;
 attribute vec2 attr_texcoords;
 attribute vec3 attr_screenworldoffset;
-attribute vec4 attr_pickcolor;
+attribute vec4 attr_t_pickcolor; // changed name for ATI, optional attribute
+                                 // cannot be first/0 index, ATI sets index
+                                 // alphabetically
 varying vec2 textureLookup ;
 varying vec3 normalizedViewCoordinate;
 varying vec4 pickcolor ;
@@ -34,5 +37,5 @@ void main()
   } else {
     fog = 1.1; // >= 1.0
   }
-  pickcolor = attr_pickcolor;
+  pickcolor = attr_t_pickcolor;
 }