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/extern/getopt.h

@@ -26,6 +26,10 @@
 #ifndef RGBDS_EXTERN_GETOPT_H
 #define RGBDS_EXTERN_GETOPT_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern char *musl_optarg;
 extern int musl_optind, musl_opterr, musl_optopt, musl_optreset;
 
@@ -43,4 +47,8 @@ int musl_getopt_long_only(int argc, char **argv, char const *optstring,
 #define required_argument  1
 #define optional_argument  2
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif