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/main.hpp

@@ -0,0 +1,59 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_MAIN_HPP
+#define RGBDS_GFX_MAIN_HPP
+
+#include <array>
+#include <filesystem>
+#include <stdint.h>
+
+#include "helpers.h"
+
+struct Options {
+	bool beVerbose = false; // -v
+	bool fixInput = false; // -f
+	bool columnMajor = false; // -h; whether to output the tilemap in columns instead of rows
+	bool allowMirroring = false; // -m
+	bool allowDedup = false; // -u
+	bool useColorCurve = false; // -C
+	uint8_t bitDepth = 2; // -d
+	uint64_t trim = 0; // -x
+	uint8_t nbPalettes = 8; // TODO
+	uint8_t nbColorsPerPal = 0; // TODO; 0 means "auto" = 1 << bitDepth;
+	std::array<uint8_t, 2> baseTileIDs{0, 0}; // TODO
+	std::array<uint16_t, 2> maxNbTiles{384, 0}; // TODO
+	std::filesystem::path tilemap{}; // -t, -T
+	std::filesystem::path attrmap{}; // -a, -A
+	std::filesystem::path palettes{}; // -p, -P
+	std::filesystem::path output{}; // -o
+	std::filesystem::path input{}; // positional arg
+
+	format_(printf, 2, 3) void verbosePrint(char const *fmt, ...) const;
+};
+
+extern Options options;
+
+void warning(char const *fmt, ...);
+void error(char const *fmt, ...);
+[[noreturn]] void fatal(char const *fmt, ...);
+
+struct Palette {
+	// An array of 4 GBC-native (RGB555) colors
+	std::array<uint16_t, 4> colors{UINT16_MAX, UINT16_MAX, UINT16_MAX, UINT16_MAX};
+
+	void addColor(uint16_t color);
+	uint8_t indexOf(uint16_t color) const;
+
+	decltype(colors)::iterator begin();
+	decltype(colors)::iterator end();
+	decltype(colors)::const_iterator begin() const;
+	decltype(colors)::const_iterator end() const;
+};
+
+#endif /* RGBDS_GFX_MAIN_HPP */