Added GetColorU32(u32) variant that does the style alpha multiply without a floating point round trip. Not 100% sure about that before the signature may seem ambiguous? But also make using imgui+drawlist more consistent in end-user extensions.

imgui.cpp

@@ -1253,6 +1253,16 @@ ImU32 ImGui::GetColorU32(const ImVec4& col)
     return ColorConvertFloat4ToU32(c); 
 }
 
+ImU32 ImGui::GetColorU32(ImU32 col)
+{ 
+    float style_alpha = GImGui->Style.Alpha;
+    if (style_alpha >= 1.0f)
+        return col;
+    int a = (col & IM_COL32_A_MASK) >> IM_COL32_A_SHIFT;
+    a = (int)(a * style_alpha); // We don't need to clamp 0..255 because Style.Alpha is in 0..1 range.
+    return (col & ~IM_COL32_A_MASK) | (a << IM_COL32_A_SHIFT);
+}
+
 // Convert rgb floats ([0-1],[0-1],[0-1]) to hsv floats ([0-1],[0-1],[0-1]), from Foley & van Dam p592
 // Optimized http://lolengine.net/blog/2013/01/13/fast-rgb-to-hsv
 void ImGui::ColorConvertRGBtoHSV(float r, float g, float b, float& out_h, float& out_s, float& out_v)
@@ -2143,6 +2153,7 @@ void ImGui::NewFrame()
     IM_ASSERT(g.IO.Fonts->Fonts.Size > 0);           // Font Atlas not created. Did you call io.Fonts->GetTexDataAsRGBA32 / GetTexDataAsAlpha8 ?
     IM_ASSERT(g.IO.Fonts->Fonts[0]->IsLoaded());     // Font Atlas not created. Did you call io.Fonts->GetTexDataAsRGBA32 / GetTexDataAsAlpha8 ?
     IM_ASSERT(g.Style.CurveTessellationTol > 0.0f);  // Invalid style setting
+    IM_ASSERT(g.Style.Alpha >= 0.0f && g.Style.Alpha <= 1.0f);  // Invalid style setting. Alpha cannot be negative (allows us to avoid a few clamps in color computations)
 
     if (!g.Initialized)
     {

imgui.h

@@ -190,6 +190,7 @@ namespace ImGui
     IMGUI_API ImVec2        GetFontTexUvWhitePixel();                                           // get UV coordinate for a while pixel, useful to draw custom shapes via the ImDrawList API
     IMGUI_API ImU32         GetColorU32(ImGuiCol idx, float alpha_mul = 1.0f);                  // retrieve given style color with style alpha applied and optional extra alpha multiplier
     IMGUI_API ImU32         GetColorU32(const ImVec4& col);                                     // retrieve given color with style alpha applied
+    IMGUI_API ImU32         GetColorU32(ImU32 col);                                             // retrieve given color with style alpha applied
 
     // Parameters stacks (current window)
     IMGUI_API void          PushItemWidth(float item_width);                                    // width of items for the common item+label case, pixels. 0.0f = default to ~2/3 of windows width, >0.0f: width in pixels, <0.0f align xx pixels to the right of window (so -1.0f always align width to the right side)