Fonts: removed misleading SizePixels >= 0.0f test and stbtt_ScaleForMappingEmToPixels() call in ImGui_ImplStbTrueType_FontSrcInit(). (#8857)

Logic was picked in 9a9712807e while extracting code from stbtt_PackFontRangesGatherRects(), but ScaleForMappingEmToPixels() was actually never called: we assert against negative SizePixels since 2015.

imgui_draw.cpp

diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 88ad029..b01e573 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -4596,10 +4596,7 @@
     if (src->MergeMode && src->SizePixels == 0.0f)
         src->SizePixels = ref_size;
 
-    if (src->SizePixels >= 0.0f)
-        bd_font_data->ScaleFactor = stbtt_ScaleForPixelHeight(&bd_font_data->FontInfo, 1.0f);
-    else
-        bd_font_data->ScaleFactor = stbtt_ScaleForMappingEmToPixels(&bd_font_data->FontInfo, 1.0f);
+    bd_font_data->ScaleFactor = stbtt_ScaleForPixelHeight(&bd_font_data->FontInfo, 1.0f);
     if (src->MergeMode && src->SizePixels != 0.0f && ref_size != 0.0f)
         bd_font_data->ScaleFactor *= src->SizePixels / ref_size; // FIXME-NEWATLAS: Should tidy up that a bit