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/proto_palette.hpp

@@ -0,0 +1,44 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_PROTO_PALETTE_HPP
+#define RGBDS_GFX_PROTO_PALETTE_HPP
+
+#include <algorithm>
+#include <array>
+#include <stddef.h>
+#include <stdint.h>
+
+class ProtoPalette {
+	// Up to 4 colors, sorted, and where SIZE_MAX means the slot is empty
+	// (OK because it's not a valid color index)
+	std::array<uint16_t, 4> _colorIndices{UINT16_MAX, UINT16_MAX, UINT16_MAX, UINT16_MAX};
+
+public:
+	/**
+	 * Adds the specified color to the set
+	 * Returns false if the set is full
+	 */
+	bool add(uint16_t color);
+
+	enum ComparisonResult {
+		NEITHER,
+		WE_BIGGER,
+		THEY_BIGGER = -1,
+	};
+	ComparisonResult compare(ProtoPalette const &other) const;
+
+	ProtoPalette &operator=(ProtoPalette const &other);
+
+	size_t size() const;
+
+	decltype(_colorIndices)::const_iterator begin() const;
+	decltype(_colorIndices)::const_iterator end() const;
+};
+
+#endif /* RGBDS_GFX_PROTO_PALETTE_HPP */