Added codec-aware encoding profiles and moved pixel format to per-variant output.

Encoders now receive the codec being used, allowing them to adjust their flags accordingly. For example, the software encoder gates x264-specific parameters behind a codec check so that future encoders like libx265 or libsvtav1 can provide their own flags without conflict. Pixel format is now specified per variant with a stream specifier rather than globally, enabling codecs like AV1 to use 10-bit color for improved compression efficiency while H.264 variants continue using yuv420p.

core/transcoder/encoder_nvenc.go

@@ -23,22 +23,16 @@ func (e *NvencEncoder) GlobalFlags() []string {
 	}
 }
 
-func (e *NvencEncoder) PixelFormat() string {
-	return pixelFormatYUV420P
-}
-
-func (e *NvencEncoder) Scaler() string {
-	return ""
-}
-
-func (e *NvencEncoder) ExtraFilters() string {
-	return ""
-}
-
 func (e *NvencEncoder) ExtraArguments() []string {
 	return nil
 }
 
+func (e *NvencEncoder) ProfileForCodec(codec string) CodecProfile {
+	return CodecProfile{
+		PixelFormat: pixelFormatYUV420P,
+	}
+}
+
 func (e *NvencEncoder) VariantFlags(v *HLSVariant) []string {
 	return []string{
 		fmt.Sprintf("-tune:v:%d", v.index), "ll",