Added function that prepares RGB565 image array to be fed into the ST7789_DrawImage function

[bayesiandog]

ST7789_DrawImage function casts the uint16_t value of the image array into a uint8_t which causes the array to be sent to the ST7789 controller low byte first. This results in wrong output and so the proposed solution is to just exchange these high and low bytes beforehand similar to the ST7789_DrawPixel function, with a different function.

issue 15

[Guilherme-Mello]

how do i use this?

[bayesiandog]

Use ST7789_PrepareRGB565_ImageArray function before calling ST7789_DrawImage.