Fix: Clear up artefacts after when using scrolling text.

[aentinger]

This fixes #27.

The following test-code was used to reproduce the problem and confirm its fix on a Arduino Uno R4 WiFi:

#include <ArduinoGraphics.h>
#include <Arduino_LED_Matrix.h>

ArduinoLEDMatrix matrix;

void setup() {
  matrix.begin();
  matrix.textScrollSpeed(100);
  matrix.textFont(Font_5x7);

  matrix.beginText(matrix.width(), 1, 80, 80, 80);
  matrix.print("H");
  matrix.endText(SCROLL_RIGHT);

  matrix.beginText(matrix.width(), 1, 80, 80, 80);
  matrix.print("H");
  matrix.endText(SCROLL_LEFT);

  matrix.beginText(matrix.width() / 2, matrix.height(), 80, 80, 80);
  matrix.print("H");
  matrix.endText(SCROLL_UP);

  matrix.beginText(matrix.width() / 2, matrix.height(), 80, 80, 80);
  matrix.print("H");
  matrix.endText(SCROLL_DOWN);
}

void loop() {}

w/ the changes contained in this PR this is the actual output on the LED matrix:

VID_20240110_085601.mp4

Note: the reason this issue only appeary when scrolling right (SCROLL_RIGHT) or down (SCROLL_DOWN) is based on the internal implementation of fonts. They are stored as a bitmap and both bottom row and right-most column are always 0, hence a auto-cleaning of artefacts takes place. In case this behaviour should change in the future with adding fonts that contain 1s in the bottom row or right-most column the appropriate artefact-cleaning implementation has already been implemented in this PR.