Mitchell Hashimoto 68318e2830 Set alpha component for fullscreen background colour (#3834)
This fixes a bug in the fullscreen behaviour on MacOS.

As per discussion #2840 native fullscreen on MacOS should set the
background to be opaque. Colours are incorrectly set due to the alpha
channel.

## Original behaviour:
When windowed:
<img width="810" alt="Screenshot 2024-12-28 at 9 02 09 PM"
src="https://github.com/user-attachments/assets/59bd4380-2744-42e6-99c7-6d7b19919206"
/>

When fullscreened:
<img width="1470" alt="Screenshot 2024-12-28 at 9 01 46 PM"
src="https://github.com/user-attachments/assets/a18f82ea-8cce-4d9b-8bb2-c279e2a753f0"
/>

## After the fix:
When fullscreened:
<img width="1470" alt="Screenshot 2024-12-28 at 9 13 17 PM"
src="https://github.com/user-attachments/assets/6c3f31ae-c206-4234-8bde-1886ce16c837"
/>
2024-12-30 10:55:48 -08:00
..
2024-12-20 14:21:59 -08:00