core: Add video dumper interface

The VideoFrame is rotated while copying, to put the pixels in their correct positions (OpenGL seems to return the image in reverted order by default)

src/core/CMakeLists.txt

@@ -36,6 +36,8 @@ add_library(core STATIC
     core.h
     core_timing.cpp
     core_timing.h
+    dumping/backend.cpp
+    dumping/backend.h
     file_sys/archive_backend.cpp
     file_sys/archive_backend.h
     file_sys/archive_extsavedata.cpp