Fixes to gloo.texture related to updating texture data

This also fixes the animate-images demo
flake
set_data reset format if necessary. Also tests.
Raise error when trying to set Texture.set_data with different dtype

To make it so that the internal dtype is changed seems a bit
complicated given the use of views and internal storage
Merge pull request #238 from almarklein/gloo-texture-fix

Fixes to gloo.texture related to updating texture data
ENH: Make backend tests independent
ENH: Add make commands
STY: Cleaner
FIX: Fix backends test
FIX: Fix for Py26
FIX: For Py26
FIX: Fix warnings
ENH: Add extra
FIX: Coveralls
FIX: Remove debug print
FIX: Fix test
ENH: Better printing
FIX: Refactor
STY: Better comment