[lib] Always load the dictionary in one go

Dictionaries larger than `ZSTD_CHUNKSIZE_MAX` used to have to be loaded
in multiple segments. Instead, when we detect large dictionaries, ensure
that we reset the context's indicies. Then, for dictionaries larger than
`ZSTD_CURRENT_MAX - 1`, only load the suffix of the dictionary. Finally,
enable DDS for large dictionaries, since we no longer load in multiple
segments.

This simplifes the dictionary loading code, and reduces opportunities
for non-determinism to slip in.

lib/compress/zstd_compress_internal.h

@@ -872,6 +872,13 @@ MEM_STATIC void ZSTD_window_clear(ZSTD_window_t* window)
     window->dictLimit = end;
 }
 
+MEM_STATIC U32 ZSTD_window_isEmpty(ZSTD_window_t const window)
+{
+    return window.dictLimit == 1 &&
+           window.lowLimit == 1 &&
+           (window.nextSrc - window.base) == 1;
+}
+
 /**
  * ZSTD_window_hasExtDict():
  * Returns non-zero if the window has a non-empty extDict.