Attempt to estimate the minimum required buffer length when initializing StreamsSequenceStream instances

For most other `DecodeStream` based streams, we'll attempt to estimate the minimum `buffer` length based on the raw stream data. The purpose of this is to avoid having to unnecessarily re-size the `buffer`, thus reducing the number of *intermediate* allocations necessary when decoding the stream data.
However, currently no such optimization is attempted for `StreamsSequenceStream`, and given that they can often be quite large that seems unfortunate. To improve this, at least somewhat, this patch utilizes the raw sizes of the `StreamsSequenceStream` sub-streams to estimate the minimum required `buffer` length.

Most likely this patch won't have a huge effect on memory consumption, however for pathological cases it should help reduce peak memory usage slightly.
One example is the PDF file in issue 2813, where currently the `StreamsSequenceStream` instances would grow their `buffer`s as `2 MiB -> 4 MiB -> 8 MiB -> 16 MiB -> 32 MiB`. With this patch, the same stream `buffers`s grow as `8 MiB -> 16 MiB -> 32 MiB`, thus avoiding a total of `12 MiB` of *intermediate* allocations (since there's two `StreamsSequenceStream` used, for rendering/text-extraction).

Author: Snuffleupagus

src/core/stream.js

@@ -126,6 +126,8 @@ var DecodeStream = (function DecodeStreamClosure() {
   var emptyBuffer = new Uint8Array(0);
 
   function DecodeStream(maybeMinBufferLength) {
+    this._rawMinBufferLength = maybeMinBufferLength || 0;
+
     this.pos = 0;
     this.bufferLength = 0;
     this.eof = false;
@@ -251,7 +253,17 @@ var DecodeStream = (function DecodeStreamClosure() {
 var StreamsSequenceStream = (function StreamsSequenceStreamClosure() {
   function StreamsSequenceStream(streams) {
     this.streams = streams;
-    DecodeStream.call(this, /* maybeLength = */ null);
+
+    let maybeLength = 0;
+    for (let i = 0, ii = streams.length; i < ii; i++) {
+      const stream = streams[i];
+      if (stream instanceof DecodeStream) {
+        maybeLength += stream._rawMinBufferLength;
+      } else {
+        maybeLength += stream.length;
+      }
+    }
+    DecodeStream.call(this, maybeLength);
   }
 
   StreamsSequenceStream.prototype = Object.create(DecodeStream.prototype);