Use single buffer mode for EInkDisplay (#34)

## Summary

* Frees up 48kB of statically allocated RAM in exchange for 48kB just
when grayscale rendering is needed

## Additional Context

* Upstream changes:
https://github.com/open-x4-epaper/community-sdk/pull/7

lib/GfxRenderer/GfxRenderer.h

@@ -12,6 +12,7 @@ class GfxRenderer {
  private:
   EInkDisplay& einkDisplay;
   RenderMode renderMode;
+  uint8_t* bwBuffer = nullptr;
   std::map<int, EpdFontFamily> fontMap;
   void renderChar(const EpdFontFamily& fontFamily, uint32_t cp, int* x, const int* y, bool pixelState,
                   EpdFontStyle style) const;
@@ -27,6 +28,8 @@ class GfxRenderer {
   static int getScreenWidth();
   static int getScreenHeight();
   void displayBuffer(EInkDisplay::RefreshMode refreshMode = EInkDisplay::FAST_REFRESH) const;
+  // EXPERIMENTAL: Windowed update - display only a rectangular region (portrait coordinates)
+  void displayWindow(int x, int y, int width, int height) const;
   void invertScreen() const;
   void clearScreen(uint8_t color = 0xFF) const;
 
@@ -49,9 +52,11 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
+  void storeBwBuffer();
+  void restoreBwBuffer();
 
   // Low level functions
   uint8_t* getFrameBuffer() const;
-  void swapBuffers() const;
+  static size_t getBufferSize();
   void grayscaleRevert() const;
 };