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.

Makefile

@@ -7,7 +7,7 @@
 #
 
 .SUFFIXES:
-.SUFFIXES: .h .y .c .o
+.SUFFIXES: .h .y .c .cpp .o
 
 # User-defined variables
 
@@ -34,10 +34,13 @@ WARNFLAGS	:= -Wall -pedantic
 
 # Overridable CFLAGS
 CFLAGS		?= -O3 -flto -DNDEBUG
+CXXFLAGS	?= -O3 -flto -DNDEBUG
 # Non-overridable CFLAGS
 # _ISOC11_SOURCE is required on certain platforms to get C11 on top of the C99-based POSIX 2008
 REALCFLAGS	:= ${CFLAGS} ${WARNFLAGS} -std=gnu11 -I include \
 		   -D_POSIX_C_SOURCE=200809L -D_ISOC11_SOURCE
+REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++17 -I include \
+		   -D_POSIX_C_SOURCE=200809L -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS		?=
 # Non-overridable LDFLAGS
@@ -102,9 +105,11 @@ rgbfix_obj := \
 	src/error.o
 
 rgbgfx_obj := \
-	src/gfx/gb.o \
+	src/gfx/convert.o \
 	src/gfx/main.o \
-	src/gfx/makepng.o \
+	src/gfx/pal_packing.o \
+	src/gfx/pal_sorting.o \
+	src/gfx/proto_palette.o \
 	src/extern/getopt.o \
 	src/error.o
 
@@ -118,7 +123,7 @@ rgbfix: ${rgbfix_obj}
 	$Q${CC} ${REALLDFLAGS} -o $@ ${rgbfix_obj} ${REALCFLAGS} src/version.c
 
 rgbgfx: ${rgbgfx_obj}
-	$Q${CC} ${REALLDFLAGS} ${PNGLDFLAGS} -o $@ ${rgbgfx_obj} ${REALCFLAGS} src/version.c ${PNGLDLIBS}
+	$Q${CXX} ${REALLDFLAGS} ${PNGLDFLAGS} -o $@ ${rgbgfx_obj} ${REALCXXFLAGS} src/version.c ${PNGLDLIBS}
 
 # Rules to process files
 
@@ -145,7 +150,10 @@ src/asm/parser.c: src/asm/parser.y
 	${BISON} $$DEFS -d ${YFLAGS} -o $@ $<
 
 .c.o:
-	$Q${CC} ${REALCFLAGS} ${PNGCFLAGS} -c -o $@ $<
+	$Q${CC} ${REALCFLAGS} -c -o $@ $<
+
+.cpp.o:
+	$Q${CXX} ${REALCXXFLAGS} ${PNGCFLAGS} -c -o $@ $<
 
 # Target used to remove all files generated by other Makefile targets
 
@@ -214,11 +222,9 @@ checkdiff:
 develop:
 	$Qenv ${MAKE} WARNFLAGS="-Werror -Wextra \
 		-Walloc-zero -Wcast-align -Wcast-qual -Wduplicated-branches -Wduplicated-cond \
-		-Wfloat-equal -Wlogical-op -Wnested-externs -Wold-style-definition \
-		-Wshift-overflow=2 \
-		-Wstrict-overflow=5 -Wstrict-prototypes -Wundef -Wuninitialized -Wunused \
+		-Wfloat-equal -Wlogical-op -Wnull-dereference -Wshift-overflow=2 \
+		-Wstringop-overflow=4 -Wstrict-overflow=5 -Wundef -Wuninitialized -Wunused \
 		-Wshadow \
-		-Wnull-dereference -Wstringop-overflow=4 \
 		-Wformat=2 -Wformat-overflow=2 -Wformat-truncation=1 \
 		-Wno-format-nonliteral \
 		-Wno-type-limits -Wno-tautological-constant-out-of-range-compare \
@@ -229,7 +235,8 @@ develop:
 		-fsanitize=signed-integer-overflow -fsanitize=bounds \
 		-fsanitize=object-size -fsanitize=bool -fsanitize=enum \
 		-fsanitize=alignment -fsanitize=null -fsanitize=address" \
-		CFLAGS="-ggdb3 -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls"
+		CFLAGS="-ggdb3 -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls" \
+		CXXFLAGS="-ggdb3 -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls"
 
 # Targets for the project maintainer to easily create Windows exes.
 # This is not for Windows users!