Implement backwards compatibility for NVENC APIs back to Video Codec SDK v11.0

This allows NVENC on drivers 456.71 (October 2020) and later.

src/nvenc/nvenc_base.h

@@ -45,6 +45,17 @@ namespace nvenc {
     bool
     nvenc_failed(NVENCSTATUS status);
 
+    /**
+     * @brief This function returns the corresponding struct version for the minimum API required by the codec.
+     * @details Reducing the struct versions maximizes driver compatibility by avoiding needless API breaks.
+     * @param version The raw structure version from `NVENCAPI_STRUCT_VERSION()`.
+     * @param v11_struct_version Optionally specifies the struct version to use with v11 SDK major versions.
+     * @param v12_struct_version Optionally specifies the struct version to use with v12 SDK major versions.
+     * @return A suitable struct version for the active codec.
+     */
+    uint32_t
+    min_struct_version(uint32_t version, uint32_t v11_struct_version = 0, uint32_t v12_struct_version = 0);
+
     const NV_ENC_DEVICE_TYPE device_type;
     void *const device;
 
@@ -68,6 +79,7 @@ namespace nvenc {
 
   private:
     NV_ENC_OUTPUT_PTR output_bitstream = nullptr;
+    uint32_t minimum_api_version = 0;
 
     struct {
       uint64_t last_encoded_frame_index = 0;