Prevent memory surge in two examples

[almarklein]

Closes #1871

The problem is that if updates are done, these are encoded as GLIR commands, which end up in a queue. You can imagine that this queue becomes a problem if it is not emptied. If a Qt window is minimized, it won't invoke a draw event, so the queue is not flushed, and it builds up.

The solution/workaround is to call canvas.context.flush() explicitly. There are multiple examples affected by this. Basically any example that does any update to a GPU buffer, texture or program. However, most these updates are so small that it does not show a noticable memory increase (no increase seen in the process manager). These two examples change so much data at such a high speed that it does matter.

@rossant here's a nasty downside of the GLIR queue :) On the long run it could be worth examining if we can get rid of the queue completely, since all our GLIR backends are local and can be flushes immediately ...

[djhoese]

Merge if you'd like