Add use_prebuilt_binding feature for consistent FFmpeg 7 API

When the system FFmpeg version doesn't match the expected API
(e.g., Ubuntu has FFmpeg 6 but rsmpeg needs FFmpeg 7 signatures),
the dynamically generated bindings will have incorrect function
signatures like *mut u8 instead of *const u8 for write_packet.

This feature allows using the prebuilt binding from src/binding.rs
which has correct FFmpeg 7 API signatures, ensuring consistent
behavior across all platforms regardless of system FFmpeg version.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

Cargo.toml

@@ -37,6 +37,9 @@ pkg-config = "0.3"
 link_system_ffmpeg = []
 # Probe and link FFmpeg with vcpkg
 link_vcpkg_ffmpeg = ["vcpkg"]
+# Use the prebuilt FFmpeg 7 bindings from src/binding.rs instead of generating at build time
+# This ensures consistent API signatures across all platforms
+use_prebuilt_binding = []
 # FFmpeg 5.* support
 ffmpeg5 = []
 # FFmpeg 6.* support

build.rs

@@ -584,6 +584,15 @@ fn docs_rs_linking(env_vars: EnvVars) {
     use_prebuilt_binding(Path::new("src/binding.rs"), binding_file_path);
 }
 
+/// When use_prebuilt_binding feature is enabled, use the prebuilt binding
+/// from src/binding.rs. This is useful when the system FFmpeg version
+/// doesn't match the expected API (e.g., system has FFmpeg 6 but we need FFmpeg 7).
+#[cfg(feature = "use_prebuilt_binding")]
+fn use_prebuilt_binding_feature(env_vars: &EnvVars) {
+    let binding_file_path = &env_vars.out_dir.as_ref().unwrap().join("binding.rs");
+    use_prebuilt_binding(Path::new("src/binding.rs"), binding_file_path);
+}
+
 fn main() {
     let env_vars = EnvVars::init();
     if env_vars.docs_rs.is_some() {
@@ -594,4 +603,9 @@ fn main() {
         // fallback to normal linking
         linking(env_vars);
     }
+
+    // If use_prebuilt_binding feature is enabled, overwrite the generated binding
+    // with our prebuilt one to ensure consistent FFmpeg 7 API signatures
+    #[cfg(feature = "use_prebuilt_binding")]
+    use_prebuilt_binding_feature(&EnvVars::init());
 }