Implement warning diagnostic flags for RGBFIX (#1766)

* Implement warning diagnostic flags for RGBFIX

* `-m/--mbc-type help` prints to stdout

* Support `-m list` as well as `-m help`

* Make invalid `rgbfix -l` characters a fatal error, like other invalid CLI arguments

* Refactor fix/main.cpp into multiple files

src/fix/warning.cpp

@@ -0,0 +1,93 @@
+#include "fix/warning.hpp"
+
+static uint32_t nbErrors;
+
+// clang-format off: nested initializers
+Diagnostics<WarningLevel, WarningID> warnings = {
+    .metaWarnings = {
+        {"all",        LEVEL_ALL       },
+        {"everything", LEVEL_EVERYTHING},
+    },
+    .warningFlags = {
+        {"mbc",        LEVEL_DEFAULT   },
+        {"overwrite",  LEVEL_DEFAULT   },
+        {"sgb",        LEVEL_DEFAULT   },
+        {"truncation", LEVEL_DEFAULT   },
+    },
+    .paramWarnings = {},
+    .state = DiagnosticsState<WarningID>(),
+};
+// clang-format on
+
+void resetErrors() {
+	nbErrors = 0;
+}
+
+uint32_t checkErrors(char const *filename) {
+	if (nbErrors > 0) {
+		fprintf(
+		    stderr,
+		    "Fixing \"%s\" failed with %u error%s\n",
+		    filename,
+		    nbErrors,
+		    nbErrors == 1 ? "" : "s"
+		);
+	}
+	return nbErrors;
+}
+
+static void incrementErrors() {
+	if (nbErrors != UINT32_MAX) {
+		++nbErrors;
+	}
+}
+
+void error(char const *fmt, ...) {
+	va_list ap;
+	fputs("error: ", stderr);
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	putc('\n', stderr);
+
+	incrementErrors();
+}
+
+void fatal(char const *fmt, ...) {
+	va_list ap;
+	fputs("FATAL: ", stderr);
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	putc('\n', stderr);
+
+	exit(1);
+}
+
+void warning(WarningID id, char const *fmt, ...) {
+	char const *flag = warnings.warningFlags[id].name;
+	va_list ap;
+
+	switch (warnings.getWarningBehavior(id)) {
+	case WarningBehavior::DISABLED:
+		break;
+
+	case WarningBehavior::ENABLED:
+		fprintf(stderr, "warning: [-W%s]\n    ", flag);
+		va_start(ap, fmt);
+		vfprintf(stderr, fmt, ap);
+		va_end(ap);
+		putc('\n', stderr);
+		break;
+
+	case WarningBehavior::ERROR:
+		fprintf(stderr, "error: [-Werror=%s]\n    ", flag);
+		va_start(ap, fmt);
+		vfprintf(stderr, fmt, ap);
+		va_end(ap);
+		putc('\n', stderr);
+
+		incrementErrors();
+		break;
+	}
+}