Reduce the Matroska read-ahead to 1KB

We only read subtitles, which are individually very small and generally
have video frames between them, resulting in most of the 65 KB
previously read being wasted. On an arbitrary test file this speeds up
subtitle reading from a cold cache by ~25%, and ~60% from warm.

Originally committed to SVN as r6716.

aegisub/src/mkv_wrap.cpp

@@ -66,7 +66,7 @@ public:
 	int error;
 };
 
-#define CACHESIZE     65536
+#define CACHESIZE     1024
 
 #ifdef __VISUALC__
 #define std_fseek _fseeki64