Fix parts of issue #27.

With this fix, alpha-blended parts get rendered correctly, but non-alpha blended parts that have alpha in their texture do not. That is weird, because the rendering code explicitly enables blending for the whole scene.
Solve issue #27 for objects using the diffuseBump shader.

Damn it, this is a silly solution. It means explicitly setting alpha to 1.0 for objects where alpha doesn't matter. But it seems to do its job.
Fixes #27

Using the same weird code in all shaders now. Damn, this is annoying stuff.
On file deletion: Use main thread (should have some OpenGL context and they're all shared); switch to default texture.
Reduce impact of #25.

This does not solve the issue completely!