Override Adafruit_GFX::drawFastHLine() and related methods for better performance.

An implementation of Adafruit_GFX is absolutely required to implement drawPixel() by overriding that pure virtual method. Everything beyond that point is optional, as they all offer fallback implementations that eventually end up at drawPixel().

For v1.0.0, the only other override was fillScreen() so there are a few promising venues for performance enhancements. Top of the list is drawFastHLine(). ESP_8_BIT (from which this project was derived) set up the frame buffer as an array of pointers to horizontal lines, so drawFastHLine() can be optimized into a single memset() call.

In contrast, drawFastVLine() is less likely to offer significant improvements, because we have to step through the line array anyway, but it might be worth a shot.

Once drawFastHLine() has been implemented, it should be tested to verify it is indeed faster and not counterintuitively slower for some surprising reason. (Requires #4 ) Once confirmed, it might be worth overriding a few other methods because their default implementation calls into drawFastVLine() instead of drawFastHLine(). (Example: fillRect()) Switching them to drawFastHLine() should be faster.

Roger-random / ESP_8_BIT_composite

Override Adafruit_GFX::drawFastHLine() and related methods for better performance. #6