Fixes to gloo.texture related to updating texture data

This also fixes the animate-images demo
flake
Added Mandelbrot set demo
Converted Mandelbrot example to vispy.app
Add panning to Mandelbrot example
Implement zoom-to-mouse in Mandelbrot example
Merge pull request #239 from jdreaver/master

Mandelbrot set demo
WIP: Better backend treatment
FIX: Fix logging test
WIP: Setting config
ENH: Add window options to Canvas
ENH: Context sharing
FIX: Read info from gl
ENH: Add SDL2
FIX: Fix SDL2
FIX: Set current
FIX: Minor fixes
FIX: Fix for GLUT
FIX: close_key
FIX: No default quit key