StbTrueTypeFont: Use fabsf instead of fabs

[Squareys]

Hey @mosra !

As discussed via gitter, the performance of StbTrueTypeFont on MSVC Debug builds is crazy bad. Through profiling I figured out that all of the time is spent in fabs() during rasterization of the Glyphs in fillGlyphCache.
In embedded stbfreetype in ImGui this is not an issue for some reason, and I found that they override STBTT_fabs(x) with fabsf(x), the float override of fabs.
This didn't entirely fix the issue (imgui is still over 10x faster at loading a font), but at least 30% speedup.

I tried to figure out if there are different compile flags applied during compilation of imgui, but that doesn't seem to be the case.

Best,
Jonathan.

[codecov-io]

Codecov Report

Merging #73 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage      91%   90.96%   -0.04%     
==========================================
  Files          53       40      -13     
  Lines        4480     3410    -1070     
==========================================
- Hits         4077     3102     -975     
+ Misses        403      308      -95

Impacted Files	Coverage Δ
.../MagnumPlugins/StbTrueTypeFont/StbTrueTypeFont.cpp	96.29% <ø> (+1%)	⬆️
...agnumPlugins/TinyGltfImporter/TinyGltfImporter.cpp	92.36% <0%> (-0.55%)	⬇️
src/Magnum/OpenDdl/Document.h	88.57% <0%> (-0.32%)	⬇️
...agnumPlugins/StanfordImporter/StanfordImporter.cpp	93.59% <0%> (-0.16%)	⬇️
...mPlugins/JpegImageConverter/JpegImageConverter.cpp	96.1% <0%> (-0.15%)	⬇️
src/Magnum/OpenDdl/OpenDdl.cpp	90.64% <0%> (-0.12%)	⬇️
src/MagnumPlugins/OpenGexImporter/openGexSpec.hpp	100% <0%> (ø)	⬆️
src/Magnum/OpenDdl/Validation.h	100% <0%> (ø)	⬆️
src/MagnumPlugins/FreeTypeFont/FreeTypeFont.cpp
...mPlugins/BasisImageConverter/BasisImageConverter.h
... and 24 more

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[mosra]

👍 for the comment explaining why

[mosra]

Thank you!

[mosra]

Hmm. This causes GCC to emit a warning:

../src/external/stb/stb_truetype.h: In function ‘void stbtt__rasterize_sorted_edges(stbtt__bitmap*, stbtt__edge*, int, int, int, int, void*)’:
../src/MagnumPlugins/StbTrueTypeFont/StbTrueTypeFont.cpp:41:30: warning: use of old-style cast to ‘float’ [-Wold-style-cast]
   41 | #define STBTT_fabs(x) fabsf(x)
      |                              ^

I'll need to investigate why it's using doubles at all. Maybe that's responsible for the remaining perf drop.