Fix animated GIF transparency and frame ghosting

[Aerhhh]

Animated GIFs produced by GifSequenceWriter had two problems that showed up together on themed tooltip renders with obfuscated text:

• Semi-transparent artwork disappeared. GIF only supports 1-bit alpha, and the JDK encoder maps every pixel with alpha below 255 to the transparent palette slot. Resource pack tooltip backgrounds are semi-transparent, so the entire background and border vanished from the encoded GIF.
• Frames stacked on top of each other. Frames were written with disposal method none, so viewers draw each frame over the previous one. With transparency in the frames, stale pixels from earlier frames stayed visible, which made animated (magic) text pile up into an unreadable smear.

Changes

• Frames are normalized before encoding: converted to TYPE_INT_ARGB, visible pixels forced fully opaque, fully transparent pixels left transparent. Caller frames are never mutated because they are reused for WebP encoding, which keeps real alpha.
• Encoder metadata is derived from TYPE_INT_ARGB to match the normalized frames; deriving it from other image types made the encoder palettize against the wrong color table and lose colors.
• Disposal method changed from none to restoreToBackgroundColor so the canvas clears between frames.

Testing

• New ImageUtilGifTest pins the encoding contract: semi-transparent pixels survive as opaque, transparent pixels stay transparent, every frame disposes to the background, input frames are not mutated, and opaque frames round-trip their colors.
• Full suite: 188 tests green.
• Verified end to end from MinecraftImageGenerator against this branch: full suite green (466 tests), and a real Hypixel-pack animated tooltip renders with the themed background intact, a transparent page background, and no ghosting between frames.