Update weather app

[gantonayde]

• Make weather icons use a palette of 3 colours.
• Remove mask from weather icons.
• Define separate Celsius to Fahrenheit function.
• Add TEMP_THRESHOLD env variable. Default value is 25 C (77 F). Above this value the temperature reading is shown in colour on colour displays.
• Add colour display weather font adjustments. Add X_OFFSET and Y_OFFSET to text. Increase text font size by 2.
• Add function that swaps two colours.
• Swap the black and colour pixels of night weather icons.
• Add DARK_MODE as environment variable. It swaps black and white pixels on the final image.

Change-type: minor Signed-off-by: Genadi Naydenov [email protected]