Use 6x8kB chunks instead of 1x48kB chunk for secondary display buffer (#36)

## Summary

- When allocating the `bwBuffer` required to restore the RED RAM in the
EPD, we were previously allocating the whole frame buffer in one
contiguous memory chunk (48kB)
- Depending on the state of memory fragmentation at the time of this
call, it may not be possible to allocate all that memory
- Instead, we now allocate 6 blocks of 8kB instead of the full 48kB,
this should mean the display updates are more resilient to different
memory conditions

## Additional Context

lib/GfxRenderer/GfxRenderer.h

@@ -10,12 +10,18 @@ class GfxRenderer {
   enum RenderMode { BW, GRAYSCALE_LSB, GRAYSCALE_MSB };
 
  private:
+  static constexpr size_t BW_BUFFER_CHUNK_SIZE = 8000;  // 8KB chunks to allow for non-contiguous memory
+  static constexpr size_t BW_BUFFER_NUM_CHUNKS = EInkDisplay::BUFFER_SIZE / BW_BUFFER_CHUNK_SIZE;
+  static_assert(BW_BUFFER_CHUNK_SIZE * BW_BUFFER_NUM_CHUNKS == EInkDisplay::BUFFER_SIZE,
+                "BW buffer chunking does not line up with display buffer size");
+
   EInkDisplay& einkDisplay;
   RenderMode renderMode;
-  uint8_t* bwBuffer = nullptr;
+  uint8_t* bwBufferChunks[BW_BUFFER_NUM_CHUNKS] = {nullptr};
   std::map<int, EpdFontFamily> fontMap;
   void renderChar(const EpdFontFamily& fontFamily, uint32_t cp, int* x, const int* y, bool pixelState,
                   EpdFontStyle style) const;
+  void freeBwBufferChunks();
 
  public:
   explicit GfxRenderer(EInkDisplay& einkDisplay) : einkDisplay(einkDisplay), renderMode(BW) {}