Move OpenGL code from GisOpenGL to objects

Move OpenGL code from GisOpenGL to objects

Add draw functions to GisObjectClass which should be set by subclasses.
This allows outside object types to be written in the normal way
(instead of always using GisCallback).

This also moves a lot of drawing code out of GisOpenGL which should
prevent it from becoming too big.

Unfortunately, there are some pesky dependency problems between
GisObject/GisViewer/GisOpenGL/GTK now. For example, Wms pulls in
GisTile, which pulls in GisViewer/GisViewer, which pulls in all of GTK.