This patch contains two different fixes. One is to change local variables from int to int64_t so that they do not cause warnings when used with ftell. There are also some casts from int64_t to int when it is known to be safe.

The second part of this patch is a function 'calculate_final_offset' that determines what the new size of an AVI file will be after a frame is written. This is used by AVI_write_frame to check if its safe to write a frame. I chose to write a separate function because I think it would be easier than cleaning up in the middle of writing a set of streams. However this may have to be done anyway in the future (see end of this description).

In the AVI_write_frame I also did some common sub-expression elimination for index values that were used multiple times.

Also, I gratuitously moved an ftell(... SEEK_END) to make another later call redundant. Added some comments for future maintainers.

I'm not sure how to test this patch, so it has not been tested.

In addition, I noticed that AVI_write_frame never checks to make sure that awrite succeeded, which means even if it does not write a file >2GB it might fail to detect other situations like permission or out-of-disk-space errors. This means that this function may need to be re-factored significantly to handle IO problems and in that case it may be better to just check for overflow along with the other errors and use the same cleanup code. In that case further rewriting of AVI_write_frame is needed and that part of this patch can be ignored.