Fix logo animation not rendering correctly on iOS

[frenzibyte]

• Closes Intro animation broken on iOS osu#9922
• Split from Update all shaders to treat colours as premultiplied #348

This issue is happening also due to premult, and is not one that can be fixed by ppy/osu-framework#6470, as premult is the norm in iOS's internals and what the logic in that PR does is use API that "un-premult" it, therefore there are inaccuracies introduced (this can be seen by the comparison picture in the PR where the two boxes aren't exactly 1:1 identical around the middle). Not to mention that this is 100% required when we use premult everywhere.

The fix is simply stripping alpha away from the textures entirely and storing its information in a colour channel instead (green is chosen), therefore the texture does not get affected by premult. This may sound one step backwards but I don't think it's usual for us to define shaders that act based on the colour value of the texture while including alpha information to it, so we have to change the formatting of the texture instead (also trimming KBs in the process).

The new format of the texture is:

• red channel for animation progression
• green channel for alpha information
• blue channel is unused
• no alpha channel present, or must be 100% in every pixel to avoid premult.

CleanShot.2024-12-23.at.16.54.09.mp4

[peppy]

Tested on windows and macOS, looks good 👍 .