Shader compilation time is included in render CPU time measurements

[Calinou]

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.