[FEAT] Animation loading | Customisable effects (countdown digits, etc)

[WillLem]

Refactored the animations loading today. Moved incidentals such as countdown digits, highlight arrow and pickup/hatch/exit numbers to the "effects" folder in styles/default. This means that they can now be customised per-style. It also means that gfx/masks now contains only essential, non-customisable graphics for rendering skill actions in-game.

LemAnimationSet's ReadData is now renamed to PrepareAnimations, and has been refactored for readability.

This has come about in part due to the way that Exit Markers are handled (still a work in progress for now, but getting to a more settled place), but also because with grenades, spears, balloon pop, Freezer overlays, invincibility overlays, etc, this procedure is getting increasingly busy. Well, that's been improved now and the code is much more streamlined. I've also renamed some variables to make it clearer what's happening throughout, particularly where the sprites are loaded.

This post is really just to log the change publicly and make a note of the commit for future reference. (Simon showed me how to rebase commits recently, and it's been an absolute game changer for my natural workflow.)

Squashed all associated commits to 0b9c489b8.