Optimize ImageLoaderSVG::create_image_from_utf8_buffer

[beru]

This PR optimizes ImageLoaderSVG::create_image_from_utf8_buffer method.

I built the engine with the following command.

python -m SCons platform=windows target=editor debug_symbols=yes

And profiled CPU usage with Microsoft Visual Studio's Perfromance Profiler by launching godot.windows.editor.x86_64.exe file. A little bif of performance improvement can be confirmed.

(Tested with AMD Ryzen 9 7900X processor)

Checked version	ImageLoaderSVG::create_image_from_utf8_buffer CPU usage (%)
Before (44fa552)	2.59
After	1.43

[fire]

That's neat and the ABGR8888S from ARGB8888S trick is pretty cool so it can directly render from lottie thorvg.

Only did an online review. Did not test.

Tested: The code is clean. Did not profile.

The colours are not scrambled which means it passes.

[clayjohn]

Looks good to me. I looked at this pretty carefully as I was surprised that such a simple change could be so beneficial.

Testing with a debug build and the icon.svg at 6x scale I was able to consistently reproduce a 30% overall improvement in re-import time(100 ms -> 70ms)

[Repiteo]

Thanks, and congratulations on your first contribution! 🎉