Allow double size sky background

[0xABADCAFE]

The engine currently uses a sky background image for outdoor areas that scrolls in sync with the player rotation. However, the graphic is only wide enough to cover 180 degrees of rotation and as a result is displayed twice for one complete rotation.

It would be a nice enhancement if the sky image could be doubled in width to allow for one full panorama. This allows for the creation of more immersive outdoor environments, for example having a fixed position on the image that would represent a global light source, such as a star or large planet. This would always be in the centre of the screen at the same player rotation value so that the lighting of exterior zones can be designed for it.

Implementation suggestion. The loading routine can detect the final uncompressed data size and set a flag to indicate if it's a vanilla size or double size and the backdrop image insertion code updated to deal with both.

[0xABADCAFE]

I plan to address this as part of the game_properties overhaul for modding. Additionally, the ability to load background images per level has been suggested.