SHP0 (morph animation) / PLT0 (GIF-compressed texture palette) support

[snailspeed3]

SHP0 and PLT0 are seldom used BRRES features, but would be nice to support for sake of completeness. Currently they are not preserved on re-save, which is certainly not ideal.

1. SHP0 -- allows for keyframing mesh properties directly, namely the position of vertices or the normal vectors of a mesh or vertex colors. Rarely used, but possibly useful for complex animations that are hard to traditionally rig.
2. PLT0 -- Specify color palettes for CI4 and CI8 textures (where "CI" means "Color Index"). Color palettes are generally inferior to traditional textures in both quality and do not support gradual mipmap transitions (trilinear filtering), so are almost never used. (CI8 is the format used by GIF files, which are notoriously artefact-y, and CI4 is substantially worse.) While the hardware supports a third format, CI14 with 14-bit color palette indices (thereby allowing color palettes with 2^14=16,384 indexable colors), the runtime library for .brres files does not.