Custom sleep screen support with BMP reading (#57)

## Summary

* Builds on top of
https://github.com/daveallie/crosspoint-reader/pull/16 - adresses
https://github.com/daveallie/crosspoint-reader/discussions/14
* This PR adds the ability for the user to supply a custom `sleep.bmp`
image at the root of the SD card that will be shown instead of the
default sleep screen if present.
* Supports:
  * Different BPPs:
    * 1bit
    * 2bit
    * 8bit
    * 24bit
    * 32bit (with alpha-channel ignored)
  * Grayscale rendering

---------

Co-authored-by: Sam Davis <sam@sjd.co>

lib/GfxRenderer/GfxRenderer.h

@@ -2,9 +2,12 @@
 
 #include <EInkDisplay.h>
 #include <EpdFontFamily.h>
+#include <FS.h>
 
 #include <map>
 
+#include "Bitmap.h"
+
 class GfxRenderer {
  public:
   enum RenderMode { BW, GRAYSCALE_LSB, GRAYSCALE_MSB };
@@ -45,6 +48,7 @@ class GfxRenderer {
   void drawRect(int x, int y, int width, int height, bool state = true) const;
   void fillRect(int x, int y, int width, int height, bool state = true) const;
   void drawImage(const uint8_t bitmap[], int x, int y, int width, int height) const;
+  void drawBitmap(const Bitmap& bitmap, int x, int y, int maxWidth, int maxHeight) const;
 
   // Text
   int getTextWidth(int fontId, const char* text, EpdFontStyle style = REGULAR) const;