AK: Remove arbitrary 1 KB limit when filling a BufferedStream's buffer

When reading, we currently only fill a BufferedStream's buffer when it
is empty, and only with 1 KB of data. This means that while the buffer
defaults to a size of 16 KB, at least 15 KB is always unused.

AK/CircularBuffer.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/CircularBuffer.h>
 #include <AK/MemMem.h>
+#include <AK/Stream.h>
 
 namespace AK {
 
@@ -189,4 +190,20 @@ ErrorOr<void> CircularBuffer::discard(size_t discarding_size)
     return {};
 }
 
+ErrorOr<size_t> CircularBuffer::fill_from_stream(Stream& stream)
+{
+    auto next_span = next_write_span();
+    if (next_span.size() == 0)
+        return 0;
+
+    auto bytes = TRY(stream.read_some(next_span));
+    m_used_space += bytes.size();
+
+    m_seekback_limit += bytes.size();
+    if (m_seekback_limit > capacity())
+        m_seekback_limit = capacity();
+
+    return bytes.size();
+}
+
 }