Tables: Angled headers: fix border position, hit box, and overflowing contents in some cases

[cfillion]

Bug 1: Border hit box extends beyond non-scrollable tables. table->AngledHeadersHeight was added to the top and bottom coordinates instead of just the top.

Bug 2: Borders remain offset after the user stops calling TableAngleHeadersRow. table->AngledHeadersHeight was not reset.

Bug 3: Table contents overflow the table's outer box when an angled header is used in combination with a list clipper. Angled header's row height could have a fractional part, while this line in ItemSize truncates window->DC.CursorMaxPos.y, leading to accumulating error at each subsequent row.

The root issue remains if the user does eg. ImGui::TableNextRow(0, 78.396423);, but this fixes the case with TableAngleHeadersRow.

This also improves rendering quality because header row's border lines are a bit fuzzy with a fractional part.

Duplication code

auto table = [] (const char *id, const bool angled, const float angle) {
  ImGuiStyle &style { ImGui::GetStyle() };
  const float oldAngle { style.TableAngledHeadersAngle };
  style.TableAngledHeadersAngle = angle * (IM_PI / 180.0f); // should be a StyleVar...
  if(!ImGui::BeginTable(id, 3, ImGuiTableFlags_Borders | ImGuiTableFlags_Resizable)) {
    style.TableAngledHeadersAngle = oldAngle;
    return;
  }
  ImGui::TableSetupColumn("Lorem Ipsum",    ImGuiTableColumnFlags_AngledHeader);
  ImGui::TableSetupColumn("Dolor Sit Amet", ImGuiTableColumnFlags_AngledHeader);
  ImGui::TableSetupColumn("Foo Bar Baz",    ImGuiTableColumnFlags_AngledHeader);
  if(angled)
    ImGui::TableAngledHeadersRow();
  ImGui::TableHeadersRow();
  for(int row = 0; row < 8; ++row) {
    ImGui::TableNextRow();
    for(int column = 0; column < 3; ++column) {
      ImGui::TableNextColumn();
      ImGui::Text("%d.%d", row, column);
    }
  }
  ImGui::EndTable();
  style.TableAngledHeadersAngle = oldAngle; // should be PopStyleVar
};

ImGui::Begin("Hello, world!");

static bool angled1 { true };
ImGui::Checkbox("Angled header##1", &angled1);
table("table1", angled1, 50.f);

static bool angled2 { true };
ImGui::Checkbox("Angled header##2", &angled2);
table(io.KeyShift ? "table1" : "table2", angled2, 35.f);

// (the following is not fixed by this PR)
if(ImGui::BeginTable("table3", 3, ImGuiTableFlags_Borders)) {
  ImGui::TableNextRow(0, 78.396423); // a possible value used by TableAngledHeadersRowEx
  ImGui::TableNextColumn();
  ImGui::Text("Foo");
  ImGuiListClipper clipper;
  clipper.Begin(32);
  while(clipper.Step()) {
    for(int row { clipper.DisplayStart }; row < clipper.DisplayEnd; ++row) {
      ImGui::TableNextRow();
      for(int column {}; column < 3; ++column) {
        ImGui::TableNextColumn();
        ImGui::Text("%.02fx%.02f", ImGui::GetCursorPosX(), ImGui::GetCursorPosY());
      }
    }
  }
  ImGui::EndTable();
}
ImGui::Text("After table");

ImGui::End();

[ocornut]

Thank you Christian for the quality PR and explanations. I've merged all three commits now.

About the clipper rounding issue, mapping the rowHeight to a slider e.g. SliderFloat("rowHeight", &rowHeight, 78.0f, 79.0f); facilitate visualizing the issue.
It mostly manifests in:

static void ImGuiListClipper_SeekCursorForItem(ImGuiListClipper* clipper, int item_n)
{
    // StartPosY starts from ItemsFrozen hence the subtraction
    // Perform the add and multiply with double to allow seeking through larger ranges
    ImGuiListClipperData* data = (ImGuiListClipperData*)clipper->TempData;
    float pos_y = (float)((double)clipper->StartPosY + data->LossynessOffset + (double)(item_n - data->ItemsFrozen) * clipper->ItemsHeight);
    ImGuiListClipper_SeekCursorAndSetupPrevLine(pos_y, clipper->ItemsHeight);
}

Where the rounding is not taken into account.

I have a bit of an issue right now changing that function because I think support for non-float should probably indeed be done elsewhere (e.g. in ItemSize ItemAdd functions), so I'll work on that later.

Thanks a lot!

(PS: I have to cherry-pick and amend with changelog etc, in vast majority of PR i don't expect user to fill in changelog as it would conflicts or be misplaced if not merged right away, so that's normal), but as I result actual PR is not marked as merged but all commits are rightly attributed to you)