This reverts commit 0f7f8f87747ae825c3b5adeae35cbb66114d2ba1, reversing
changes made to 9f1505435877cf8f5abe7b66fb2717c7e39c1988.

and clean up a bit.

CustomTexture: prefetch all available textures

There is no nice way to correctly "detect" the "used" memory, so we just say
we're fine to use 50% of the physical memory for custom textures.

This will fix out-of-memory crashes, but we still might run into swapping issues.

Just return std::string as this code isn't performance critical.

Various fixes to formatting and whitespace

Google fixed the issue of posix_memalign not being available. It now works fine in r10d of the NDK.