Some simple functions for manipulating display buffer/partial updates

[PaulKlinger]

Hi,

I've added some simple functions I needed for a project:

• lcd_goto_xpix_y to go to a specific x coordinate instead of a character
• lcd_clear_buffer to clear the display buffer
• lcd_check_buffer to read a pixel value from the buffer
• lcd_display_block to send only part of the buffer to the display (this can increase performance a lot if only a small part is updated)

Feel free to close if you don't think these would be useful.