ImDrawListSplitter: create first draw cmd on demand + Internals: fix incorrect ImBitArraySetBitRange() (only used by tables)

Make it cheaper to allocate unused draw cmd, can't measure perf difference other our stress tests.

imgui_draw.cpp

@@ -1417,6 +1417,7 @@ void ImDrawListSplitter::ClearFreeMemory()
 
 void ImDrawListSplitter::Split(ImDrawList* draw_list, int channels_count)
 {
+    IM_UNUSED(draw_list);
     IM_ASSERT(_Current == 0 && _Count <= 1 && "Nested channel splitting is not supported. Please use separate instances of ImDrawListSplitter.");
     int old_channels_count = _Channels.Size;
     if (old_channels_count < channels_count)
@@ -1441,12 +1442,6 @@ void ImDrawListSplitter::Split(ImDrawList* draw_list, int channels_count)
             _Channels[i]._CmdBuffer.resize(0);
             _Channels[i]._IdxBuffer.resize(0);
         }
-        if (_Channels[i]._CmdBuffer.Size == 0)
-        {
-            ImDrawCmd draw_cmd;
-            ImDrawCmd_HeaderCopy(&draw_cmd, &draw_list->_CmdHeader); // Copy ClipRect, TextureId, VtxOffset
-            _Channels[i]._CmdBuffer.push_back(draw_cmd);
-        }
     }
 }
 
@@ -1536,8 +1531,10 @@ void ImDrawListSplitter::SetCurrentChannel(ImDrawList* draw_list, int idx)
     draw_list->_IdxWritePtr = draw_list->IdxBuffer.Data + draw_list->IdxBuffer.Size;
 
     // If current command is used with different settings we need to add a new command
-    ImDrawCmd* curr_cmd = &draw_list->CmdBuffer.Data[draw_list->CmdBuffer.Size - 1];
-    if (curr_cmd->ElemCount == 0)
+    ImDrawCmd* curr_cmd = (draw_list->CmdBuffer.Size == 0) ? NULL : &draw_list->CmdBuffer.Data[draw_list->CmdBuffer.Size - 1];
+    if (curr_cmd == NULL)
+        draw_list->AddDrawCmd();
+    else if (curr_cmd->ElemCount == 0)
         ImDrawCmd_HeaderCopy(curr_cmd, &draw_list->_CmdHeader); // Copy ClipRect, TextureId, VtxOffset
     else if (ImDrawCmd_HeaderCompare(curr_cmd, &draw_list->_CmdHeader) != 0)
         draw_list->AddDrawCmd();

imgui_internal.h

@@ -461,7 +461,7 @@ inline void          ImBitArraySetBitRange(ImU32* arr, int n, int n2)
     while (n <= n2)
     {
         int a_mod = (n & 31);
-        int b_mod = ((n2 >= n + 31) ? 31 : (n2 & 31)) + 1;
+        int b_mod = (n2 > (n | 31) ? 31 : (n2 & 31)) + 1;
         ImU32 mask = (ImU32)(((ImU64)1 << b_mod) - 1) & ~(ImU32)(((ImU64)1 << a_mod) - 1);
         arr[n >> 5] |= mask;
         n = (n + 32) & ~31;