- schedule texture key change to next update() iteration

[maho]

This commit fixes problem with flickering.

Testcase - https://github.com/mahomahomaho/kivent-memory-issue-case

Before this pull request:
0. - frame is rendered properly

1. • 1sec later, texture_set is set to next texture, frame gets blank
2. • 1sec later, frame is rendered properly
3. goto 0.

after pull request, whole texture_set is scheduled and executed in the beginning of Gameworld.update()

It's very likely that coding style is against your coding style, then please give guidelines or fix yourself.

[Kovak]

The only thing I don't like about this PR is the naming of schedule_once. It A: is not very descriptive, it should be something like schedule_before_update_callback or something else descriptive of what it does. and B: The name shadows Clock.schedule_once which is a bit confusing.