Remove extern "C" functions from wasm module

We don't need these functions and their export. They account for ~20kb in
the optimized .wasm - plus JS glue code.

sixtyfps_runtime/rendering_backends/default/lib.rs

@@ -69,6 +69,7 @@ pub use default_backend::{
 
 #[doc(hidden)]
 #[cold]
+#[cfg(not(target_arch = "wasm32"))]
 pub fn use_modules() {
     default_backend::use_modules();
     #[cfg(feature = "sixtyfps-rendering-backend-qt")]
@@ -77,6 +78,7 @@ pub fn use_modules() {
     sixtyfps_rendering_backend_gl::use_modules();
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub mod ffi {
     use sixtyfps_corelib::window::ffi::ComponentWindowOpaque;
     use sixtyfps_corelib::window::ComponentWindow;

sixtyfps_runtime/rendering_backends/gl/lib.rs

@@ -1399,6 +1399,7 @@ pub fn create_gl_window_with_canvas_id(canvas_id: String) -> ComponentWindow {
 
 #[doc(hidden)]
 #[cold]
+#[cfg(not(target_arch = "wasm32"))]
 pub fn use_modules() {
     sixtyfps_corelib::use_modules();
 }

sixtyfps_runtime/rendering_backends/qt/lib.rs

@@ -31,6 +31,7 @@ mod key_generated;
 
 #[doc(hidden)]
 #[cold]
+#[cfg(not(target_arch = "wasm32"))]
 pub fn use_modules() -> usize {
     sixtyfps_corelib::use_modules() + {
         #[cfg(no_qt)]