Writing an input decoder plugin

Input plugins (AMP_PLUGIN_INPUT, type 2) add new file formats. Implement amp_plugin_input_can_decode(ext), amp_plugin_input_open(path,&sr,&ch), amp_plugin_input_read(handle,buf,maxFrames) returning frames produced (0 = EOF), and amp_plugin_input_close(handle). Emit interleaved float PCM. Start from templates/input-plugin-template.c in the SDK.