Dropping requirement for 8bpp normal zoom -- 32bpp-only graphics

[peter1138]

Hi,

Soon nmlc will drop the requirement that the first sprite is 8bpp normal zoom, which will allow the creation if 32bpp-only extra-zoom basesets and NewGRFs.

It turns out this restriction was only ever coded into nmlc. Neither OpenTTD nor grfcodec prevent it, although until recently grfcodec couldn't decode such files.

ldpl's grf-py system also already allows creation of 32bpp-only files.

This change will mean that authors no longer need to create dummy empty or blank 8bpp sprites, which will make their lives easier.

[Aegir]

Hurray!