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/Bitmap.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <FS.h>
+
+enum class BmpReaderError : uint8_t {
+  Ok = 0,
+  FileInvalid,
+  SeekStartFailed,
+
+  NotBMP,
+  DIBTooSmall,
+
+  BadPlanes,
+  UnsupportedBpp,
+  UnsupportedCompression,
+
+  BadDimensions,
+  PaletteTooLarge,
+
+  SeekPixelDataFailed,
+  BufferTooSmall,
+  OomRowBuffer,
+  ShortReadRow,
+};
+
+class Bitmap {
+ public:
+  static const char* errorToString(BmpReaderError err);
+
+  explicit Bitmap(File& file) : file(file) {}
+  BmpReaderError parseHeaders();
+  BmpReaderError readRow(uint8_t* data, uint8_t* rowBuffer) const;
+  BmpReaderError rewindToData() const;
+  int getWidth() const { return width; }
+  int getHeight() const { return height; }
+  bool isTopDown() const { return topDown; }
+  bool hasGreyscale() const { return bpp > 1; }
+  int getRowBytes() const { return rowBytes; }
+
+ private:
+  static uint16_t readLE16(File& f);
+  static uint32_t readLE32(File& f);
+
+  File& file;
+  int width = 0;
+  int height = 0;
+  bool topDown = false;
+  uint32_t bfOffBits = 0;
+  uint16_t bpp = 0;
+  int rowBytes = 0;
+  uint8_t paletteLum[256] = {};
+};