Upload the textures from the main thread.

The texture upload from the dequeue threads (and then using fences) _should_
work, but there are seemingly driver bugs galore here; AMDs drivers crash
outright, while NVIDIA's are much more subtle, crashing after only days,
and running under apitrace shifts the behavior enough that I don't see it.

Let's try to do all of this from the main thread instead; Nageru is still
multithreaded OpenGL in many places, but with some luck, it will the last of
the NVIDIA crashes (generally manifesting themselves as libGL.so.1 free()ing
some stray pointer that was never actually allocated).