+ fix minor clang-tidy warnings which seems reasonable

Windows: Amend 6b0cf2e6 to facilitate working in viewport branch + handle safe area padding and ConfigWindowsMoveFromTitleBarOnly.

Popups: Changed 'int mouse_buttons' to ImGuiPopupFlags. Added ImGuiPopupFlags_NoOpenOverExistingPopup, ImGuiPopupFlags_NoOpenOverItems. Refactored signature of BeginPopupContextWindow().

Popups: Added ImGuiPopupFlags type, ImGuiPopupFlags_AnyPopupId and ImGuiPopupFlags_AnyPopupLevel flags for IsPopupOpen().

# Conflicts:
#	docs/CHANGELOG.txt

BeginPopupModal() doesn't set the ImGuiWindowFlags_NoSavedSettings flag anymore, and will not always be auto-centered. (#915, #3091)

Internals: Initialize drawlist earlier in Begin() to facilitate detecting accidental draw earlier than legal. (#3311)

As a side-effect, some rounding error may be neutralized however this isn't the intent. (#3309)

Clang: Reduce uses of __has_warning for overall sanity, as compilers are hostile to software targetting multiple compiler version.

Popups: Fix BeginPopupContextVoid() when clicking over the area made unavailable by a modal. (#1636)

Popups: Fix an edge case where programatically closing a popup while clicking on its empty space would attempt to focus it and close other popups. (#2880)

Internals: Allow ItemHoverable() to be used with id==0 to facilitate high-level read-only hover test in widget code.

Renamed OpenPopupOnItemClick() to OpenPopupContextItem(). Kept inline redirection function (will obsolete). + Removed CalcItemRectClosestPoint() entry point

Columns: Lower overhead on column switches and switching to background channel. (second attempt for 9b3ce494)

Internals: Bits, comments, added ImRect::ToVec4()

ImDrawList: Fixed minor bug introduced in 1.75 where AddCircle() with 12 segments would generate an extra unrequired vertex.

Amend 5363af7f, 051ce076

Revert "Columns: Lower overhead on column switches and switching to background channel (some stress tests in debug builds went 3->2 ms). (#125)"

This reverts commit 9b3ce494.

Columns: Lower overhead on column switches and switching to background channel (some stress tests in debug builds went 3->2 ms). (#125)

This change benefits Columns but was primarily made with Tables in mind.

ImDrawList, ImDrawListSplitter, Columns: Fixed an issue where changing channels with different TextureId, VtxOffset would incorrectly apply new settings to draw channels. (#3129, #3163)

ImDrawList: Clarifying and guarateeing that CmdBuffer.back()->UserCallback should be always be NULL.

ImDrawList: Prefixed internal functions with underscore, renamed UpdateClipRect() to _OnChangedClipRect(), UpdateTextureID() -> _OnChangedTextureID()

ImDrawList: Store header/current ImDrawCmd in instance to simplify merging code. Amend 0320e725, toward #3163, #3129