WIP - Support for TWO UV channels

As per the [forum request / discussion](http://pioneerspacesim.net/forum/viewtopic.php?f=3&t=199&p=2283#p2283) for supporting an ambient occlusion bake texture and dual UV sets.

This moves the pattern textures from texture3/4 onto texture4/5.
Adds an ambient texture.
Multiplies the lighting results by this new ambient channel texture.
Adds `ATTRIB_UV1` type and extends loading to support it and fill out the extended vertex format.
* this DOUBLES the size of the vertex format and pads it out with wasted space :(

Ping @Luomu