Move setting GL interpolation into _UpdateTexture
Move setting GL interpolation into _UpdateTexture

If OpenGL is called only from drawing functions, visvis can be used
safely in a threaded environment.  The interpolate property of
BaseTexture was calling OpenGL functions directly; now it just sets
a property and lets _UpdateTexture (called during the drawing process)
set the interpolation.

chrisidefix/visvis,pbfy0/visvis.dev,pbfy0/visvis,pbfy0/visvis,AlanZatarain/visvis.dev,Alwnikrotikz/visvis,Alwnikrotikz/visvis.dev,chrisidefix/visvis,almarklein/visvis.dev,Alwnikrotikz/visvis,chrisidefix/visvis,Alwnikrotikz/visvis,pbfy0/visvis,chiluf/visvis.dev