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/defaultinitalloc.hpp

@@ -0,0 +1,38 @@
+/**
+ * Allocator adaptor that interposes construct() calls to convert value-initialization
+ * (which is what you get with e.g. `vector::resize`) into default-initialization (which does not
+ * zero out non-class types).
+ * From https://stackoverflow.com/questions/21028299/is-this-behavior-of-vectorresizesize-type-n-under-c11-and-boost-container/21028912#21028912
+ */
+
+#ifndef DEFAULT_INIT_ALLOC_H
+#define DEFAULT_INIT_ALLOC_H
+
+#include <memory>
+#include <vector>
+
+template<typename T, typename A = std::allocator<T>>
+class default_init_allocator : public A {
+	using a_t = std::allocator_traits<A>;
+public:
+	template<typename U>
+	struct rebind {
+		using other = default_init_allocator<U, typename a_t::template rebind_alloc<U>>;
+	};
+
+	using A::A; // Inherit the allocator's constructors
+
+	template<typename U>
+	void construct(U * ptr) noexcept(std::is_nothrow_default_constructible_v<U>) {
+		::new(static_cast<void *>(ptr)) U;
+	}
+	template<typename U, typename... Args>
+	void construct(U * ptr, Args && ... args) {
+		a_t::construct(static_cast<A &>(*this), ptr, std::forward<Args>(args)...);
+	}
+};
+
+template<typename T>
+using DefaultInitVec = std::vector<T, default_init_allocator<T>>;
+
+#endif