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>

src/activities/boot_sleep/SleepActivity.h

@@ -1,8 +1,14 @@
 #pragma once
 #include "../Activity.h"
 
+class Bitmap;
+
 class SleepActivity final : public Activity {
  public:
   explicit SleepActivity(GfxRenderer& renderer, InputManager& inputManager) : Activity(renderer, inputManager) {}
   void onEnter() override;
+
+ private:
+  void renderDefaultSleepScreen() const;
+  void renderCustomSleepScreen(const Bitmap& bitmap) const;
 };