Reduce zero initialization and copying overhead of render commands

[hrydgard]

This introduces yet another custom vector, this time one specialized for cheaply adding items that don't need to be fully initialized, like our graphics command unions. It simply has a push_uninitialized() method that returns a pointer to the new item. std::vector simply can't do that.

This saves both zero-initialization of the structs and copying of the structs during push_back to the vectors.

So, this new vector is now used for command lists in both OpenGL and Vulkan backends. OpenGL gets the biggest wins here, sometimes up to a 8% detectable performance increase when profiling on PC (!). Vulkan only gets maybe 1-2% since those command structs are generally smaller I guess, and there are less of them since more state is passed in other ways like in pipeline keys.

Not intending to merge this until after the 1.15 process.

[unknownbrackets]

Maybe this should have an assert or something? I don't think anything's using this right now, though.

I do wonder if there might've been a way to coax the compiler to do less copying with enough move/emplace, but this is probably safer. I assume it's also faster in debug anyway...

-[Unknown]

[hrydgard]

I actually tried a bunch of things, but when looking at the disassembly, still lots of zeroing and copying. So gave up and did it this way. And yes, it helps debug performance indeed.