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/platform/renderer.cc

@@ -88,6 +88,8 @@ void Renderer::Init(const mjModel* model) {
       mjrFilamentConfig render_config;
       mjrf_defaultFilamentConfig(&render_config);
       render_config.native_window = native_window_;
+      render_config.width = model->vis.global.offwidth;
+      render_config.height = model->vis.global.offheight;
       render_config.force_software_rendering = IsSoftware(gfx_);
       render_config.graphics_api =
           IsOpenGl(gfx_) || IsWebGl(gfx_) ? mjGFX_OPENGL : mjGFX_VULKAN;