Fix #30315: Temporary proxy files are not erased and old proxys are not updated if the proxy is built more then once (windows)

Two issues are fixed here:
- On windows rename() doesn't actually renames file if destination file is already exist.
  Solved by ulinking previously built proxy/timecode.
- IMB_TC_MAX_SLOT was set to incorrect value leading to record run timecode calculating
  several times.

source/blender/imbuf/IMB_imbuf.h

@@ -199,7 +199,7 @@ typedef enum IMB_Timecode_Type {
 				  device (*every* consumer camcorder can do
 				  that :) )*/
 	IMB_TC_RECORD_RUN_NO_GAPS = 8,
-	IMB_TC_MAX_SLOT   = 8
+	IMB_TC_MAX_SLOT   = 4
 } IMB_Timecode_Type;
 
 typedef enum IMB_Proxy_Size {

source/blender/imbuf/intern/indexer.c

@@ -147,6 +147,7 @@ void IMB_index_builder_finish(anim_index_builder * fp, int rollback)
 	if (rollback) {
 		unlink(fp->temp_name);
 	} else {
+		unlink(fp->name);
 		rename(fp->temp_name, fp->name);
 	}
 
@@ -687,6 +688,7 @@ static void free_proxy_output_ffmpeg(struct proxy_output_ctx * ctx,
 	} else {
 		get_proxy_filename(ctx->anim, ctx->proxy_size, 
 		                   fname, FALSE);
+		unlink(fname);
 		rename(fname_tmp, fname);
 	}
 	
@@ -1043,6 +1045,7 @@ static void index_rebuild_fallback(struct anim * anim,
 			if (*stop) {
 				unlink(fname_tmp);
 			} else {
+				unlink(fname);
 				rename(fname_tmp, fname);
 			}
 		}