gpu: remove usingnamespace for callbacks.zig and types.zig

[michael-brennan2005]

Continuing off of issue #796 and pull request #797, I removed the usingnamespaces for callbacks.zig and types.zig and added their types.

I wrote up a basic python script for this: it looks for all pub consts or pub fns in the file and then generates the re-exports. It only writes one @import per each file and then uses a variable to do the re-exports, but I did a simple find and replace for @imports on each declaration, to keep it unified with what #797 did.

Here's the script if this could be useful for removing more usingnamespace: https://gist.github.com/tech0tron/e26d54d057e6c030b1d773dcf3b5350c

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[emidoots]

Excellent work, thank you!