Benchmarks: Fix compile error caused by custom std::string allocator.

[mwl4]

It is possible to set custom allocator for std::string in Include/RmlUi/Config/Config.h:

using String = std::basic_string<char, std::char_traits<char>, MyAllocator<char>>;

ankerl::nanobench::Bench::run expects as first parameter either char const* or std::string const&.
With custom allocator, String type cannot be implicitly converted to std::string.

[mikke89]

Thanks again! I'm curios to ask, is there any specific reason you are running the benchmarks with your own config file?

[mwl4]

@mikke89 As comment in Config.h says:

 * This file provides the means to configure various types used across RmlUi. It is possible to override container
 * types with your own, provided they are compatible with STL, or customize STL containers, for example by setting
 * custom allocators.

If I am allowed to override container type with my own, then performance of the whole library will be different. It is good to know how different.

[mikke89]

That's a very good reason! Do you have some results you could share perhaps, found anything that improves it significantly?

[mwl4]

During frame profiling I have noticed that updating data model triggers update that takes 6 ms on high-spec hardware. In most of the samples (callstacks) allocation is apparent. I walked through the code, and there is a lot of strings involved in layout update.

For now I will try two things:

1. larger String SSO capacity - MSVC std::string implementation allows up to 15 characters being held in SSO buffer. I can try to implement my own String with larger buffer and see if it helps anything.
2. custom allocator for String - Rml is single threaded library, allocator might take advantage of this fact and return required buffer faster than standard operator new.