Reinitialize the result_buffer

When using the displayio's `fill_area` function, it does not copy anything for pixels where no Group, TileGrid, or VectorIO is present.  In this library, this was causing previous values in the result_buffer to be recopied to the output bitmap file whenever a "un-owned" pixel is observed.

This reinitializes the `result_buffer` to be sure that any "un-owned" pixels are made black in the output bitmap file.

Resolves issue:
#8

Author: kmatch98

adafruit_bitmapsaver.py

@@ -91,6 +91,7 @@ def _write_pixels(output_file, pixel_source, palette):
                     color >>= 8
                     buffer_index += 1
         else:
+            result_buffer = bytearray(2048)
             data = pixel_source.fill_row(y - 1, result_buffer)
             for i in range(width):
                 pixel565 = (data[i * 2] << 8) + data[i * 2 + 1]