inverted colors (monochrome)

[primvla]

Hey, im not a programmer, but i needed an 'active' state for my gui elements, so i've hacked your lib with a few lines here:

SSD1306Ascii.h:

• at initialization: SSD1306Ascii() : m_magFactor(1), m_font(0), m_invert(false) {}
• somewhere near set2X(): void invert() {m_invert = !m_invert;}
• and at the bottom vars: bool m_invert;

and here SSD1306Ascii.cpp, function write:

• after the uint8_t b = readFontByte(base + c + r*w); line: if (m_invert) b = 0xFF - b;
• and replaced the ssd1306WriteRamBuf(0); few lines below with uint8_t v = m_invert ? 0xFF : 0; ssd1306WriteRamBuf(v);

Then within my sketch i can turn on and off this inverted colors like such:

oled.println("Hello world!."); // white on black
oled.invert();
oled.println("inverted"); // black on white
oled.invert();
oled.println("A long line may be truncated"); // white on black again

I am aware this is a dirty dirty hack, but you could include a similar functionality in the library since it doesn't take much space and is potentially quite useful...

cheers!