Fix Adafruit_GFX writeColor implementation for ESP32 (#2712)

Fixes #2711

The problem occurs with a code path for ESP32 which buffers up to 32 pixels at a time for transfer. However, the first call to 'writePixels' clears the buffer to black (0 - empty response data). If the fill colour is black no abnormal operation is observed, but any other colour and at most 32 pixels get filled with the requested colour.

The benefit of these optimisations is minimal so the library is patched to simplify code path for all architectures.