Bug with clamping values in DragScalar if limits the same

[DimaKoltun]

float minValue = -180.0f;
float maxValue = 1.0f;

{
    ImGui::Begin("Test DragScalar");

    ImGui::DragFloat("Min Limit", &minValue, 1.0f, -180.0f, maxValue, "%.3f", ImGuiSliderFlags_AlwaysClamp);
    ImGui::DragFloat("Max Limit", &maxValue, 1.0f, minValue, 180.0f, "%.3f", ImGuiSliderFlags_AlwaysClamp);

    ImGui::End();
}

If you change "Max Limit" to -180 then you can make "Min Limit" lower than -180, so PR fix this problem

[ocornut]

Unfortunately this would also pretty much break the basic assumption of DragXXX functions:
Use v_min < v_max to clamp edits to given limits
Where default values are v_min = v_max = 0.0f.

However, since last time this has been discussed we now have flags available in the API, I think we can add one to specify always using those values.

[ocornut]

(Please note that the current workaround is to use ImGuiSliderFlags_ReadOnly or ImGuiItemFlags_ReadOnly for now. But I aim to fix this better)

[ocornut]

I have pushed b3c8747 which introduces:

• Added ImGuiSliderFlags_ClampZeroRange
• Added ImGuiSliderFlags_ClampOnInput (old behavior of _AlwaysClamp)
• Changed ImGuiSliderFlags_AlwaysClamp to = ImGuiSliderFlags_ClampOnInput | ImGuiSliderFlags_ClampZeroRange

I have changed default logic to assume clamping when v_min==v_max with values != 0.0f
For values == 0.0f, ImGuiSliderFlags_ClampZeroRange is required due to legacy reason.