Added clearPixel(x,y) function (#247)

Author: szczys

src/OLEDDisplay.cpp

@@ -101,6 +101,17 @@ void OLEDDisplay::setPixel(int16_t x, int16_t y) {
   }
 }
 
+void OLEDDisplay::clearPixel(int16_t x, int16_t y) {
+  if (x >= 0 && x < this->width() && y >= 0 && y < this->height()) {
+    switch (color) {
+      case BLACK:   buffer[x + (y / 8) * this->width()] |=  (1 << (y & 7)); break;
+      case WHITE:   buffer[x + (y / 8) * this->width()] &= ~(1 << (y & 7)); break;
+      case INVERSE: buffer[x + (y / 8) * this->width()] ^=  (1 << (y & 7)); break;
+    }
+  }
+}
+
+
 // Bresenham's algorithm - thx wikipedia and Adafruit_GFX
 void OLEDDisplay::drawLine(int16_t x0, int16_t y0, int16_t x1, int16_t y1) {
   int16_t steep = abs(y1 - y0) > abs(x1 - x0);

src/OLEDDisplay.h

@@ -140,6 +140,9 @@ class OLEDDisplay : public Print {
     // Draw a pixel at given position
     void setPixel(int16_t x, int16_t y);
 
+    // Clear a pixel at given position FIXME: INVERSE is untested with this function
+    void clearPixel(int16_t x, int16_t y);
+
     // Draw a line from position 0 to position 1
     void drawLine(int16_t x0, int16_t y0, int16_t x1, int16_t y1);