Avoid unnecessary zeroing out in na-mainloop

[JohnScience]

As I was assessing the maturity of Rust for mobile development, I tried different things.

I used na-mainloop example as a template for https://github.com/JohnScience/rust_android_native_activity.

While I was working on it, I noticed a few bad practices:

• using one huge unsafe block instead of a few smaller ones. This creates problems with identifying the safety requirements.
• zeroing out before initializing. AFAIK, this kind of code can't be optimized by the compiler.

My take on it is here.

My code is worse because it assumes R8G8B8A8_UNORM but "It works on my machine" TM and I've noticed there's a pull request with some code that ensures R8G8B8A8_UNORM buffer format, and I'm likely to use it soon.