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.

src/CMakeLists.txt

@@ -70,9 +70,13 @@ set(rgbfix_src
     )
 
 set(rgbgfx_src
-    "gfx/gb.c"
-    "gfx/main.c"
-    "gfx/makepng.c"
+    "gfx/convert.cpp"
+    "gfx/main.cpp"
+    "gfx/pal_packing.cpp"
+    "gfx/pal_sorting.cpp"
+    "gfx/proto_palette.cpp"
+    "extern/getopt.c"
+    "error.c"
     )
 
 set(rgblink_src
@@ -97,6 +101,8 @@ foreach(PROG "asm" "fix" "gfx" "link")
   install(TARGETS rgb${PROG} RUNTIME DESTINATION bin)
 endforeach()
 
+set_target_properties(rgbgfx PROPERTIES CXX_STANDARD 17 CXX_STANDARD_REQUIRED True)
+
 if(LIBPNG_FOUND) # pkg-config
   target_include_directories(rgbgfx PRIVATE ${LIBPNG_INCLUDE_DIRS})
   target_link_directories(rgbgfx PRIVATE ${LIBPNG_LIBRARY_DIRS})