glir: Don't draw when count is zero

Fixes issue #661
Refactoring of GLIR queues: 1 on each canvas.context

So now there is a queue on each gloo object, which is kept there.
The queue can be associated with other queues, creating a sort
of tree. This also got rid of the need for a shared queue.
Added shared context example
address review comments

also canvas(config, shared)
attempt to reduce segfaults
minor fixes
appveyor fix
ENH: Add size test override
Merge pull request #672 from Eric89GXL/size-override

ENH: Add size test override
Merge pull request #653 from almarklein/glir-queues

Glir queue refactoring
Merge pull request #674 from almarklein/issue661-2

glir: Don't draw when count is zero
WIP: py.test
FIX: Fix testing
FIX: Fix
FIX: Keep nose for skippers
FIX: Fix name
FIX: Fix for skipping under py.test
FIX: Fix coverage
STY: Change name to "unit"
Merge pull request #673 from Eric89GXL/pytest

ENH: py.test