gfx.py does not check for .pal files when building .2bpp

[PikalaxALT]

When rebuilding .2bpp files in pokecrystal/gfx/pics, gfx.py does not take into account the presence of a .pal file. Instead, it completely overwrites the existing file, resulting in a garbage build. It also overwrites the bitmask.asm and frames.asm files, which breaks pic animations.