Shader compilation time is included in render CPU time measurements

CPU time is much lower on a second run in 3D benchmarks, which indicates an issue with shader compilation times being counted in CPU time. See https://github.com/godotengine/godot-benchmarks/pull/22#issuecomment-1483572938 where this was originally reported.

This may be an engine issue, but I'm not sure if it can be fixed within the engine. So we may have to work around this issue in the benchmarks project, e.g. by ignoring more frames at the beginning of the benchmark run.

Start with the 3D benchmarks project and compare CPU measurements between the first and second runs. Read the original report in pull request 22's linked comment, then determine whether the engine or benchmark setup is responsible. Done means shader compilation time is no longer counted in reported render CPU time.

Written by the indexing model from the issue text.