vcompress: make vcomp_pop_retry placeholder const

To protect from accidental errors like overriding it with a different
value, possibly also returning a reference to it and subsequent
destruction.

Author: MartinPulec

src/video_compress.cpp

@@ -71,7 +71,7 @@ using namespace std;
 struct compress_state;
 
 static video_frame pop_retry_frame{};
-shared_ptr<video_frame> vcomp_pop_retry(&pop_retry_frame, [](video_frame *){});
+const shared_ptr<video_frame> vcomp_pop_retry(&pop_retry_frame, [](video_frame *){});
 
 namespace {
 /**

src/video_compress.h

@@ -103,7 +103,7 @@ std::shared_ptr<video_frame> compress_pop(struct compress_state *);
 #include <vector>
 
 /// _pop callbacks can return to indicate recoverable error
-extern std::shared_ptr<video_frame> vcomp_pop_retry;
+extern const std::shared_ptr<video_frame> vcomp_pop_retry;
 
 /**
  * @brief Initializes video compression