Implement preliminary version of "reverse" feature

Not hooked to all RGBGFX flags yet, but good enough for most use cases
(and as a base for future development, should I need to `reset --hard`.)

TODOs marked appropriately.

src/CMakeLists.txt

@@ -75,6 +75,7 @@ set(rgbgfx_src
     "gfx/pal_sorting.cpp"
     "gfx/process.cpp"
     "gfx/proto_palette.cpp"
+    "gfx/reverse.cpp"
     "gfx/rgba.cpp"
     "extern/getopt.c"
     "error.c"