Fix Windows-breaking use of struct vs class

MSVC's (broken) ABI breaks otherwise.
What the f@!$#ck, Microsoft?
(Thank you based Clang for warning, by the way.)

include/gfx/pal_packing.hpp

@@ -16,7 +16,7 @@
 
 #include "gfx/main.hpp"
 
-class Palette;
+struct Palette;
 class ProtoPalette;
 
 namespace packing {

include/gfx/pal_sorting.hpp

@@ -17,7 +17,7 @@
 
 #include "gfx/rgba.hpp"
 
-class Palette;
+struct Palette;
 
 namespace sorting {