Fixed TitleBg/TitleBgActive color being rendered above WindowBg color, being inconsistent and causing visual artefact (#655)

Broke the meaning of TitleBg and TitleBgActive. Only affect values where
Alpha<1.0f. Fixed default theme.

imgui.cpp

@@ -153,6 +153,14 @@
  Here is a change-log of API breaking changes, if you are using one of the functions listed, expect to have to fix some code.
  Also read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2016/05/12 (1.49) - title bar (using TitleBg/TitleBgActive colors) isn't rendered over a window background (WindowBg color) anymore. 
+                       If your TitleBg/TitleBgActive alpha was 1.0f or you are using the default theme it will not affect you. However if your TitleBg/TitleBgActive alpha was <1.0f you need to tweak your custom theme to readjust for the fact that we don't draw a WindowBg background behind the title bar.
+                       This helper function will convert an old TitleBg/TitleBgActive color into a new one with the same visual output, given that color and the WindowBg color.
+                           ImVec4 ConvertTitleBgCol(const ImVec4& win_bg_col, const ImVec4& title_bg_col)
+                           {
+                               float new_a = 1.0f - ((1.0f - win_bg_col.w) * (1.0f - title_bg_col.w)), k = title_bg_col.w / new_a;
+                               return ImVec4((win_bg_col.x * win_bg_col.w + title_bg_col.x) * k, (win_bg_col.y * win_bg_col.w + title_bg_col.y) * k, (win_bg_col.z * win_bg_col.w + title_bg_col.z) * k, new_a);
+                           }
  - 2016/05/07 (1.49) - removed confusing set of GetInternalState(), GetInternalStateSize(), SetInternalState() functions. Now using CreateContext(), DestroyContext(), GetCurrentContext(), SetCurrentContext().
  - 2016/05/02 (1.49) - renamed SetNextTreeNodeOpened() to SetNextTreeNodeOpen(), no redirection.
  - 2016/05/01 (1.49) - obsoleted old signature of CollapsingHeader(const char* label, const char* str_id = NULL, bool display_frame = true, bool default_open = false) as extra parameters were badly designed and rarely used. You can replace the "default_open = true" flag in new API with CollapsingHeader(label, ImGuiTreeNodeFlags_DefaultOpen).
@@ -746,9 +754,9 @@ ImGuiStyle::ImGuiStyle()
     Colors[ImGuiCol_FrameBg]                = ImVec4(0.80f, 0.80f, 0.80f, 0.30f);   // Background of checkbox, radio button, plot, slider, text input
     Colors[ImGuiCol_FrameBgHovered]         = ImVec4(0.90f, 0.80f, 0.80f, 0.40f);
     Colors[ImGuiCol_FrameBgActive]          = ImVec4(0.90f, 0.65f, 0.65f, 0.45f);
-    Colors[ImGuiCol_TitleBg]                = ImVec4(0.50f, 0.50f, 1.00f, 0.45f);
+    Colors[ImGuiCol_TitleBg]                = ImVec4(0.27f, 0.27f, 0.54f, 0.83f);
     Colors[ImGuiCol_TitleBgCollapsed]       = ImVec4(0.40f, 0.40f, 0.80f, 0.20f);
-    Colors[ImGuiCol_TitleBgActive]          = ImVec4(0.50f, 0.50f, 1.00f, 0.55f);
+    Colors[ImGuiCol_TitleBgActive]          = ImVec4(0.32f, 0.32f, 0.63f, 0.87f);
     Colors[ImGuiCol_MenuBarBg]              = ImVec4(0.40f, 0.40f, 0.55f, 0.80f);
     Colors[ImGuiCol_ScrollbarBg]            = ImVec4(0.20f, 0.25f, 0.30f, 0.60f);
     Colors[ImGuiCol_ScrollbarGrab]          = ImVec4(0.40f, 0.40f, 0.80f, 0.30f);
@@ -4056,7 +4064,7 @@ bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_us
                 bg_color.w = bg_alpha;
             bg_color.w *= style.Alpha;
             if (bg_color.w > 0.0f)
-                window->DrawList->AddRectFilled(window->Pos, window->Pos+window->Size, ColorConvertFloat4ToU32(bg_color), window_rounding);
+                window->DrawList->AddRectFilled(window->Pos+ImVec2(0,window->TitleBarHeight()), window->Pos+window->Size, ColorConvertFloat4ToU32(bg_color), window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? 15 : 4|8);
 
             // Title bar
             if (!(flags & ImGuiWindowFlags_NoTitleBar))