avutil/executor: Allowing thread_count be zero

Before the patch, disable threads support at configure/build time
was the only method to force zero thread in executor. However,
it's common practice for libavcodec to run on caller's thread when
user specify thread number to one. And for WASM environment, whether
threads are supported needs to be detected at runtime. So executor
should support zero thread at runtime.

A single thread executor can be useful, e.g., to handle network
protocol. So we can't take thread_count one as zero thread, which
disabled a valid usercase.

Other libraries take -threads 0 to mean auto. Executor as a low
level utils doesn't do cpu detect. So take thread_count zero as
zero thread, literally.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavutil/executor.h

@@ -46,7 +46,7 @@ typedef struct AVTaskCallbacks {
 /**
  * Alloc executor
  * @param callbacks callback structure for executor
- * @param thread_count worker thread number
+ * @param thread_count worker thread number, 0 for run on caller's thread directly
  * @return return the executor
  */
 AVExecutor* av_executor_alloc(const AVTaskCallbacks *callbacks, int thread_count);