Custom sprite pivot

This update introduces an useful feature: the ability to set the sprite's pivot position with TLN_SetSpritePivot.  Until now it was always fixed to top-left corner and scaled from the center, but now it can be set freely inside its bounding rect. This pivot point sets the exact placement and scaling source. For gameplay configurations that use some kind of 3D projection -like 3/4 or racing roads-, it's much more natural to pivot sprites at the bottom middle, where the "ground" is:
http://www.tilengine.org/doc/md_sprites.html#autotoc_md93

• Left: default pivot at top left
• Right: custom pivot at bottom center

Quick layer enable

Another enhancement on this update is that a layer can be enabled again with TLN_EnableLayer after it has been disabled with TLN_DisableLayer, as long as it was previously configured
http://www.tilengine.org/doc/md_layers.html#autotoc_md39