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.

CMakeLists.txt

@@ -10,7 +10,7 @@
 cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
 
 project(rgbds
-        LANGUAGES C)
+        LANGUAGES C CXX)
 
 # get real path of source and binary directories
 get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
@@ -45,14 +45,14 @@ else()
     # A non-zero optimization level is desired in debug mode, but allow overriding it nonetheless
     # TODO: this overrides anything previously set... that's a bit sloppy!
     set(CMAKE_C_FLAGS_DEBUG "-g -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls" CACHE STRING "" FORCE)
+    set(CMAKE_CXX_FLAGS_DEBUG "-g -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls" CACHE STRING "" FORCE)
   endif()
 
   if(MORE_WARNINGS)
     add_compile_options(-Werror -Wextra
                         -Walloc-zero -Wcast-align -Wcast-qual -Wduplicated-branches -Wduplicated-cond
-                        -Wfloat-equal -Wlogical-op -Wnested-externs -Wnull-dereference
-                        -Wold-style-definition -Wshift-overflow=2 -Wstrict-overflow=5
-                        -Wstrict-prototypes -Wstringop-overflow=4 -Wundef -Wuninitialized -Wunused
+                        -Wfloat-equal -Wlogical-op -Wnull-dereference -Wshift-overflow=2
+                        -Wstringop-overflow=4 -Wstrict-overflow=5 -Wundef -Wuninitialized -Wunused
                         -Wshadow # TODO: -Wshadow=compatible-local ?
                         -Wformat=2 -Wformat-overflow=2 -Wformat-truncation=1
                         -Wno-format-nonliteral # We have a couple of "dynamic" prints