Ensure consistent imgui enum types when OR-ing

[slowriot]

This fixes #688 and similar issues.

This was a latent bug going back to 2021, now breaks with modern Clang diagnostics and most recent Imgui master.

ImPlot is building plot_button_flags by OR-ing values from two different Dear ImGui enum families:

• public ImGuiButtonFlags_ in include/imgui/imgui.h:1865: AllowOverlap, MouseButtonLeft/Right/Middle
• internal ImGuiButtonFlagsPrivate_ in include/imgui/imgui_internal.h:1031: PressedOnClick, PressedOnDoubleClick, FlattenChildren

ButtonBehavior() still takes ImGuiButtonFlags, but the bitwise expression itself mixes distinct enum types, so Clang emits -Wenum-enum-conversion, and as warnings are treated as errors, the build fails.

This PR casts enums to a consistent type when OR-ing them together.

[Copilot]

Pull request overview

This PR resolves build failures triggered by newer Clang diagnostics (and recent Dear ImGui changes) by avoiding bitwise-OR operations between different enum types (public vs internal ImGui button flag enums, and similarly mixed ImPlot flag enums in examples/demo).

Changes:

• Casts the first operand in ImGui button-flag bitwise expressions to ImGuiButtonFlags to prevent -Wenum-enum-conversion when combining public and private flag enums.
• Updates ButtonBehavior flag expressions used for plots, legends, and subplot splitters to use the consistent cast pattern.
• Adjusts ImPlot spec/demo examples that OR item flags with specialized item-type flags to start from an ImPlotItemFlags-typed value.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
implot.h	Updates documentation examples to avoid mixed-enum OR by casting to ImPlotItemFlags.
implot.cpp	Fixes mixed ImGui public/private enum OR-ing by casting to ImGuiButtonFlags before combining flags.
implot_demo.cpp	Updates demo code to avoid mixed-enum OR by casting to ImPlotItemFlags.

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