moved rendering to a system.

To ensure that the rendering system is being run in the main thread, I
added the concept of "Sync systems", so that when a system group is
created it's possible to specify whether it's possible to run it on a
separate thread or not.

build.zig

@@ -118,6 +118,7 @@ pub fn build(b: *std.Build) void {
         .root_module = mods,
         .name = "mods",
     });
+
     const mods_docs = b.addInstallDirectory(.{
         .source_dir = mods_lib.getEmittedDocs(),
         .install_dir = .prefix,