Fonts: (Internal) Added undocumented ImFontBaked::LoadNoRenderOnLayout. (#8758, #8465)

Amend fd75bdccb0.

imgui.h

@@ -3745,9 +3745,10 @@ struct ImFontBaked
     // [Internal] Members: Cold
     float                       Ascent, Descent;    // 4+4   // out // Ascent: distance from top to bottom of e.g. 'A' [0..FontSize] (unscaled)
     unsigned int                MetricsTotalSurface:26;// 3  // out // Total surface in pixels to get an idea of the font rasterization/texture cost (not exact, we approximate the cost of padding between glyphs)
-    unsigned int                WantDestroy:1;      // 0     //     // Queued for destroy
-    unsigned int                LoadNoFallback:1;   // 0     //     // Disable loading fallback in lower-level calls.
-    int                         LastUsedFrame;      // 4     //     // Record of that time this was bounds
+    unsigned int                WantDestroy:1;         // 0  //     // Queued for destroy
+    unsigned int                LoadNoFallback:1;      // 0  //     // Disable loading fallback in lower-level calls.
+    unsigned int                LoadNoRenderOnLayout:1;// 0  //     // Enable a two-steps mode where CalcTextSize() calls will load AdvanceX *without* rendering/packing glyphs. Only advantagous if you know that the glyph is unlikely to actually be rendered, otherwise it is slower because we'd do one query on the first CalcTextSize and one query on the first Draw.
+    int                         LastUsedFrame;         // 4  //     // Record of that time this was bounds
     ImGuiID                     BakedId;            // 4     //     // Unique ID for this baked storage
     ImFont*                     ContainerFont;      // 4-8   // in  // Parent font
     void*                       FontLoaderDatas;    // 4-8   //     // Font loader opaque storage (per baked font * sources): single contiguous buffer allocated by imgui, passed to loader.

imgui_draw.cpp

@@ -4483,7 +4483,7 @@ static ImFontGlyph* ImFontBaked_BuildLoadGlyph(ImFontBaked* baked, ImWchar codep
 
 static float ImFontBaked_BuildLoadGlyphAdvanceX(ImFontBaked* baked, ImWchar codepoint)
 {
-    if (baked->Size >= IMGUI_FONT_SIZE_THRESHOLD_FOR_LOADADVANCEXONLYMODE)
+    if (baked->Size >= IMGUI_FONT_SIZE_THRESHOLD_FOR_LOADADVANCEXONLYMODE || baked->LoadNoRenderOnLayout)
     {
         // First load AdvanceX value used by CalcTextSize() API then load the rest when loaded by drawing API.
         float only_advance_x = 0.0f;