video_core/shader: Minor changes

These can inhibit the ability of a compiler to perform RVO.

Allows the function to be non-allocating in terms of the output string.

Same behavior, but without the need to heap allocate

This is behaviorally equivalent and also fixes a bug where some members
weren't being moved over.

Keeps them alphabetically sorted for readability.

Provides logical symmetry to the interface.

Avoids unnecessary atomic reference count increments and decrements.

Provides const equivalents of data accessors for use within const
contexts.

Shader_IR: Implement a full control flow decompiler for the shader IR.

TextureCache: Add the ability to deduce if two textures are depth on blit.

SDL: Fix missing header

This fixes linux and mingw builds.

Port citra-emu/citra#4950: "Add FPS to SDL title bar"

[crypto] Use IsAllZeroArray helper function

externals: Track mainline zlib as a submodule