ImDrawList: clarified that PathArcTo()/PathArcToFast() cannot take radius < 0.0f. (#3491) + changed poor-man ceiling in _CalcCircleAutoSegmentCount() to use 0.999999f to reduce gaps

Previously it sorts of accidentally worked but would lead to counter-clockwise paths which and have an effect on anti-aliasing.

docs/CHANGELOG.txt

@@ -42,6 +42,8 @@ Breaking Changes:
       bool closed = false ----> use ImDrawFlags_None, or 0
       bool closed = true  ----> use ImDrawFlags_Closed
   Difference may not be noticeable for most but zealous type-checking tools may report a need to change.
+- ImDrawList: clarified that PathArcTo()/PathArcToFast() won't render with radius < 0.0f. Previously it sorts
+  of accidentally worked but would lead to counter-clockwise paths which and have an effect on anti-aliasing.
 - Style: renamed rarely used style.CircleSegmentMaxError (old default = 1.60f)
   to style.CircleTessellationMaxError (new default = 0.30f) as its meaning changed. (#3808) [@thedmd]
 - Win32+MinGW: Re-enabled IME functions by default even under MinGW. In July 2016, issue #738 had me incorrectly

imgui.cpp

@@ -376,6 +376,7 @@ CODE
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2021/03/11 (1.82) - clarified that ImDrawList::PathArcTo(), ImDrawList::PathArcToFast() won't render with radius < 0.0f. Previously it sorts of accidentally worked but would generally lead to counter-clockwise paths and have an effect on anti-aliasing.
  - 2021/03/10 (1.82) - upgraded ImDrawList::AddPolyline() and PathStroke() "bool closed" parameter to "ImDrawFlags flags". The matching ImDrawFlags_Closed value is guaranteed to always stay == 1 in the future.
  - 2021/02/22 (1.82) - win32+mingw: Re-enabled IME functions by default even under MinGW. In July 2016, issue #738 had me incorrectly disable those default functions for MinGW. MinGW users should: either link with -limm32, either set their imconfig file  with '#define IMGUI_DISABLE_WIN32_DEFAULT_IME_FUNCTIONS'.
  - 2021/02/17 (1.82) - renamed rarely used style.CircleSegmentMaxError (old default = 1.60f) to style.CircleTessellationMaxError (new default = 0.30f) as the meaning of the value changed.

imgui_draw.cpp

@@ -545,7 +545,7 @@ void ImDrawList::_OnChangedVtxOffset()
 int ImDrawList::_CalcCircleAutoSegmentCount(float radius) const
 {
     // Automatic segment count
-    const int radius_idx = (int)(radius + 0.999f); // ceil to never reduce accuracy
+    const int radius_idx = (int)(radius + 0.999999f); // ceil to never reduce accuracy
     if (radius_idx < IM_ARRAYSIZE(_Data->CircleSegmentCounts))
         return _Data->CircleSegmentCounts[radius_idx]; // Use cached value
     else
@@ -1026,9 +1026,10 @@ void ImDrawList::AddConvexPolyFilled(const ImVec2* points, const int points_coun
     }
 }
 
+// 0: East, 3: South, 6: West, 9: North, 12: East
 void ImDrawList::PathArcToFast(const ImVec2& center, float radius, int a_min_of_12, int a_max_of_12)
 {
-    if (radius == 0.0f)
+    if (radius <= 0.0f)
     {
         _Path.push_back(center);
         return;
@@ -1052,7 +1053,7 @@ void ImDrawList::PathArcToFast(const ImVec2& center, float radius, int a_min_of_
 
 void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, float a_max, int num_segments)
 {
-    if (radius == 0.0f)
+    if (radius <= 0.0f)
     {
         _Path.push_back(center);
         return;