Refactor FilamentContext to prepare for upcoming changes.

Separate out the parts of the FilamentContext initialization that
depends on the mjModel into a separate Init function.

PiperOrigin-RevId: 890373341
Change-Id: I06b524668cd60dad804d437958f4f6606c46200b

src/experimental/filament/render_context_filament.cc

@@ -49,7 +49,8 @@ void mjrf_makeFilamentContext(const mjModel* m, mjrContext* con,
   if (g_filament_context != nullptr) {
     mju_error("Context already exists!");
   }
-  g_filament_context = new mujoco::FilamentContext(config, m);
+  g_filament_context = new mujoco::FilamentContext(config);
+  g_filament_context->Init(m);
 }
 
 void mjrf_defaultContext(mjrContext* con) {
@@ -60,6 +61,8 @@ void mjrf_makeContext(const mjModel* m, mjrContext* con, int fontscale) {
   mjr_freeContext(con);
   mjrFilamentConfig cfg;
   mjrf_defaultFilamentConfig(&cfg);
+  cfg.width = m->vis.global.offwidth;
+  cfg.height = m->vis.global.offheight;
   mjrf_makeFilamentContext(m, con, &cfg);
 }