Integrate the Tracy profiler

[inequation]

This is now good enough to discuss how (and if) it should be merged, I think.

Tracy is a pretty great frame profiler: https://wolf.nereid.pl/projects/tracy-profiler/

Instructions for use:

• set TRACY_ENABLE=ON in your CMake cache and build,
• on Windows, running the game as admin is required for the sampling profiler to work.
• consult the Tracy user manual: https://github.com/wolfpld/tracy/releases/download/v0.13.1/tracy.pdf

I see two major points of contention:

• whether using a git submodule for Tracy is appropriate, and
• whether the approach I've taken - always include Tracy headers (they #define empty macros that boil all instrumentation away), regardless of whether TRACY_ENABLE is on/true or not, is the desired one.

There's also the question of whether sys_tracy.cpp lives in the right place of the source tree vs which target it's part of (tracy), missing documentation, code style etc.

I'm also considering upstreaming it into ioquake3, but that's a bunch of additional work.

[smallmodel]

I think the maximum would be to just integrate it in the build in the least invasive way possible without touching any C/C++ source file (except for initializing or cleaning up).
OPM should not be hardly dependent on a specific profiler, anyone should be able to bring their own.

godotengine/godot#104851 Someone integrated Tracy to Godot Engine, but in a way that makes it soft-dependent.