Send only the changed dots of a frame to the Nano

Frames were sent as F0 plus one s command per yellow dot (thousands of
characters). During scrolling that overran the Nano's serial buffer, and
half-received frames redrew the whole screen. showBitmap() now sends two B
commands over the changed rectangle (dots off, dots on), or one B command for
a full frame, and the pacing counts the characters sent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/BROSE9323/BROSE9323.cpp

@@ -23,6 +23,38 @@ void BROSE9323::begin(Stream *s)
 {
   stream = s;
 }
+
+size_t BROSE9323::showBitmap(const uint8_t *bitmap, bool changes_only, bool instant)
+{
+  // What the Nano shows now, in drawBitmap format (the buffers keep the bits the other
+  // way around)
+  uint8_t *shown = (uint8_t *)malloc(_buffer_size);
+  if (!shown)
+    return 0;
+  for (uint16_t i = 0; i < _buffer_size; i++)
+  {
+    uint8_t b = _old_buffer[i], r = 0;
+    for (uint8_t k = 0; k < 8; k++)
+      if (b & (1 << k))
+        r |= 0x80 >> k;
+    shown[i] = r;
+  }
+  size_t sent = frameCommands(bitmap, changes_only ? shown : nullptr, width(), height(), instant,
+                              [this](const char *text) { stream->print(text); });
+  free(shown);
+
+  for (uint8_t y = 0; y < height(); y++)
+    for (uint8_t x = 0; x < width(); x++)
+    {
+      uint8_t &byte = _new_buffer[y * _buffer_width + x / 8];
+      if (bitmap[y * _buffer_width + x / 8] & (0x80 >> (x & 7)))
+        byte |= 1 << (x & 7);
+      else
+        byte &= ~(1 << (x & 7));
+    }
+  memcpy(_old_buffer, _new_buffer, _buffer_size);
+  return sent;
+}
 #else
 void BROSE9323::begin(void)
 {