enable viewport window background alpha and rounding

Temporary patch until ocornut/imgui ocornut#2766 is merged.

imgui.cpp

@@ -6058,8 +6058,6 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
             ImU32 bg_col = GetColorU32(GetWindowBgColorIdx(window));
             if (window->ViewportOwned)
             {
-                // No alpha
-                bg_col = (bg_col | IM_COL32_A_MASK);
                 if (is_docking_transparent_payload)
                     window->Viewport->Alpha *= DOCKING_TRANSPARENT_PAYLOAD_ALPHA;
             }
@@ -6726,7 +6724,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
         // Lock window rounding for the frame (so that altering them doesn't cause inconsistencies)
         // Large values tend to lead to variety of artifacts and are not recommended.
-        if (window->ViewportOwned || window->DockIsActive)
+        if (window->DockIsActive)
             window->WindowRounding = 0.0f;
         else
             window->WindowRounding = (flags & ImGuiWindowFlags_ChildWindow) ? style.ChildRounding : ((flags & ImGuiWindowFlags_Popup) && !(flags & ImGuiWindowFlags_Modal)) ? style.PopupRounding : style.WindowRounding;
@@ -13158,8 +13156,8 @@ void ImGui::WindowSyncOwnedViewport(ImGuiWindow* window, ImGuiWindow* parent_win
     // We can also tell the backend that clearing the platform window won't be necessary,
     // as our window background is filling the viewport and we have disabled BgAlpha.
     // FIXME: Work on support for per-viewport transparency (#2766)
-    if (!(window_flags & ImGuiWindowFlags_NoBackground))
-        viewport_flags |= ImGuiViewportFlags_NoRendererClear;
+    // if (!(window_flags & ImGuiWindowFlags_NoBackground) && window->WindowRounding == 0.0f)
+    //     viewport_flags |= ImGuiViewportFlags_NoRendererClear;
 
     window->Viewport->Flags = viewport_flags;