(Ozone) Ensure all assets have 'white' transparent pixels

Created by: jdgleaver

While debugging this: https://github.com/libretro/RetroArch/issues/10317, I discovered a tiresome issue with the way that textures are rendered.

Whenever a texture is scaled, the pixel values get interpolated - and at any transparent edges, the colours of the transparent pixels themselves bleed into the visible area. If the source texture files are correctly formatted, this is not a problem - but it turns out that many (most?) images in this repo have transparent pixels with the wrong colour. These transparent pixels are mostly black, so when the interpolation takes place, all transparent edges end up with an ugly dark border.

This PR corrects all of Ozone's assets, setting the colour of any transparent pixels to white. This fixes a host of subtle rendering errors.