Reimplement basic RGBGFX features in C++

Currently missing from the old version:
- `-f` ("fixing" the input image to be indexed)
- `-m` (the code for detecting mirrored tiles is missing, but all of the
        "plumbing" is otherwise there)
- `-C`
- `-d`
- `-x` (though I need to check the exact functionality the old one has)
- Also the man page is still a draft and needs to be fleshed out

More planned features are not implemented yet either:
- Explicit palette spec
- Better error messages, also error "images"
- Better 8x16 support, as well as other "dedup unit" sizes
- Support for arbitrary number of palettes & colors per palette
- Other output formats (for example, a "full" palette map for "streaming"
  use cases like gb-open-world)
- Quantization?

Some things may also be bugged:
- Transparency support
- Tile offsets (not exposed yet)
- Tile counts per bank (not exposed yet)

...and performance remains to be checked.
We need to set up some tests, honestly.

include/gfx/pal_sorting.hpp

@@ -0,0 +1,26 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_PAL_SORTING_HPP
+#define RGBDS_GFX_PAL_SORTING_HPP
+
+#include <png.h>
+#include <vector>
+
+class Palette;
+
+namespace sorting {
+
+void indexed(std::vector<Palette> &palettes, int palSize, png_color const *palRGB,
+             png_byte *palAlpha);
+void grayscale(std::vector<Palette> &palettes);
+void rgb(std::vector<Palette> &palettes);
+
+}
+
+#endif /* RGBDS_GFX_PAL_SORTING_HPP */