Add RACK_SYSTEM_DIR and RACK_USER_DIR environment variables on all OS's to override asset paths.

src/asset.cpp

@@ -38,6 +38,13 @@ static void initSystemDir() {
 		return;
 	}
 
+	// Environment variable overrides
+	const char* env = getenv("RACK_SYSTEM_DIR");
+	if (env) {
+		systemDir = env;
+		return;
+	}
+
 #if defined ARCH_MAC
 	CFBundleRef bundle = CFBundleGetMainBundle();
 	assert(bundle);
@@ -85,6 +92,13 @@ static void initUserDir() {
 		return;
 	}
 
+	// Environment variable overrides
+	const char* env = getenv("RACK_USER_DIR");
+	if (env) {
+		userDir = env;
+		return;
+	}
+
 #if defined ARCH_WIN
 	// Get "My Documents" path
 	wchar_t documentsBufW[MAX_PATH] = L".";