Pipeline/shader race-condition-during-shutdown crash fix

[hrydgard]

If you exit the game while shaders are still compiling, we could end up in a situation where we cleared the shader cache while pipeline creation (using said shaders) was still going on on the worker pool, especially on Android where pipeline creation can be notoriously slow.

Now, kill the pipeline cache first, and while doing so, block the shutdown process until all pending shaders are finished compiling.

Potentially if many are queued we could add a cancelling facility, but likely not worth the trouble.

These crashes weren't too damaging to the play experience since they don't happen while you're actually playing, only when leaving the game, but this pollutes the Google Play crash logs, and the app exiting when you exit a game is a bit unexpected.

Also, sneak in a leak fix on image view creation failure, although very likely very obscure. I have seen a crash that seems connected though.