river: add custom wlroots log handler

This makes river's log output more consistent and will allow
filtering by the wlroots scope in the future.

river/wlroots_log_wrapper.c

@@ -0,0 +1,41 @@
+#include <assert.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+#include <wlr/util/log.h>
+
+#define BUFFER_SIZE 1024
+
+void river_wlroots_log_callback(enum wlr_log_importance importance, const char *ptr, size_t len);
+
+static void callback(enum wlr_log_importance importance, const char *fmt, va_list args) {
+	char buffer[BUFFER_SIZE];
+
+	// Need to make a copy of the args in case our buffer isn't big
+	// enough and we need to use them again.
+	va_list args_copy;
+	va_copy(args_copy, args);
+
+	const int length = vsnprintf(buffer, BUFFER_SIZE, fmt, args);
+	// Need to add one for the terminating 0 byte
+	if (length + 1 <= BUFFER_SIZE) {
+		// The formatted string fit within our buffer, pass it on to river
+		river_wlroots_log_callback(importance, buffer, length);
+	} else {
+		// The formatted string did not fit in our buffer, we need
+		// to allocate enough memory to hold it.
+		char *allocated_buffer = malloc(length + 1);
+		if (allocated_buffer != NULL) {
+			const int length2 = vsnprintf(allocated_buffer, length + 1, fmt, args_copy);
+			assert(length2 == length);
+			river_wlroots_log_callback(importance, allocated_buffer, length);
+			free(allocated_buffer);
+		}
+	}
+
+	va_end(args_copy);
+}
+
+void river_init_wlroots_log(enum wlr_log_importance importance) {
+	wlr_log_init(importance, callback);
+}