Read in larger chunks from SD at a time

This helps a little with the stuttering. Some kind of readahead is
probably the ideal tho.

src/codecs/source_buffer.cpp

@@ -18,7 +18,8 @@
 namespace codecs {
 
 [[maybe_unused]] static constexpr char kTag[] = "dec_buf";
-static constexpr size_t kBufferSize = 1024 * 8;
+static constexpr size_t kBufferSize = 1024 * 128;
+static constexpr size_t kReadThreshold = 1024 * 8;
 
 SourceBuffer::SourceBuffer()
     : buffer_(reinterpret_cast<std::byte*>(
@@ -34,7 +35,7 @@ SourceBuffer::~SourceBuffer() {
 }
 
 auto SourceBuffer::Refill(IStream* src) -> bool {
-  if (bytes_in_buffer_ == buffer_.size_bytes()) {
+  if (bytes_in_buffer_ > kReadThreshold) {
     return false;
   }
   bool eof = false;