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/error.h

@@ -12,10 +12,18 @@
 #include "helpers.h"
 #include "platform.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void warn(char const NONNULL(fmt), ...) format_(printf, 1, 2);
 void warnx(char const NONNULL(fmt), ...) format_(printf, 1, 2);
 
 _Noreturn void err(char const NONNULL(fmt), ...) format_(printf, 1, 2);
 _Noreturn void errx(char const NONNULL(fmt), ...) format_(printf, 1, 2);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* RGBDS_ERROR_H */