feat(ffi): add processing_poll_events for external event loop integration

Exposes a C FFI function that ticks Bevy's app one frame, allowing
embedders that manage their own window and event loop (e.g. a native
C++ host using GLFW) to advance libprocessing's internal state each
frame without using the built-in GLFW runner.

The pattern is:
  processing_init();
  processing_surface_create(...);
  processing_graphics_create(...);
  while (running) {
      running = processing_poll_events();
      processing_begin_draw(gfx);
      // draw calls
      processing_end_draw(gfx);
  }

Adds processing_core as a direct dependency of processing_ffi since
app_mut is not re-exported through processing::prelude.

Author: pepc84

crates/processing_ffi/Cargo.toml

@@ -19,6 +19,7 @@ cuda = ["processing/cuda"]
 
 [dependencies]
 processing = { workspace = true }
+processing_core = { workspace = true }
 bevy = { workspace = true }
 half = "2.7"
 

crates/processing_ffi/src/lib.rs

@@ -4437,21 +4437,25 @@ fn key_code_to_u32(kc: KeyCode) -> u32 {
     }
 }
 
-/// Tick the Bevy app one frame and poll GLFW events.
+/// Tick the Bevy app one frame.
 /// Call this once per frame from your external event loop, before begin_draw.
 /// Returns true if the app should continue running, false if it should exit.
 ///
+/// This is intended for embedders that drive their own window and event loop
+/// (e.g. a native C++ host) and need to advance libprocessing's internal state
+/// each frame without using the built-in GLFW runner.
+///
 /// SAFETY:
-/// - processing_init has been called.
-/// - This is called from the same thread as init.
+/// - `processing_init` has been called.
+/// - This is called from the same thread as `init`.
+/// - This must not be called after `processing_exit`.
 #[unsafe(no_mangle)]
 pub extern "C" fn processing_poll_events() -> bool {
     error::clear_error();
-    let result = processing_core::app_mut(|app| {
+    match processing_core::app_mut(|app| {
         app.update();
         Ok(())
-    });
-    match result {
+    }) {
         Ok(_) => true,
         Err(_) => false,
     }