Warn about rgbgfx -O without -o or any of -A -T -P -Q

src/gfx/main.cpp

@@ -44,6 +44,8 @@ static struct LocalOptions {
 	bool autoPalmap;
 	bool groupOutputs;
 	bool reverse;
+
+	bool autoAny() const { return autoAttrmap || autoTilemap || autoPalettes || autoPalmap; }
 } localOptions;
 
 // Short options
@@ -846,6 +848,16 @@ int main(int argc, char *argv[]) {
 		);
 	}
 
+	if (localOptions.groupOutputs) {
+		if (!localOptions.autoAny()) {
+			warnx("Grouping outputs ('-O') is enabled, but without any automatic output paths "
+			      "('-A', '-P', '-Q', or '-T')");
+		}
+		if (options.output.empty()) {
+			warnx("Grouping outputs ('-O') is enabled, but without an output tile data file ('-o')"
+			);
+		}
+	}
 	auto autoOutPath = [](bool autoOptEnabled, std::string &path, char const *extension) {
 		if (!autoOptEnabled) {
 			return;

test/gfx/warn_group_output.err

@@ -0,0 +1,2 @@
+warning: Grouping outputs ('-O') is enabled, but without any automatic output paths ('-A', '-P', '-Q', or '-T')
+warning: Grouping outputs ('-O') is enabled, but without an output tile data file ('-o')

test/gfx/warn_group_output.flags

@@ -0,0 +1 @@
+-O