[brenocq]

Fixes #715

Building the example with a restricted IMPLOT_CUSTOM_NUMERIC_TYPES, e.g. -DIMPLOT_CUSTOM_NUMERIC_TYPES="(float)(double)" (the way implot.h suggests for shrinking the binary), compiles implot_demo.cpp but then fails to link. A few demos hardcode their data as ImS8, int or unsigned int to show that PlotXxx() is not limited to float/double, and ImPlot only instantiates PlotXxx<T> for the types in IMPLOT_NUMERIC_TYPES:

undefined reference to `void ImPlot::PlotBars<signed char>(char const*, signed char const*, int, double, double, ImPlotSpec const&)'
undefined reference to `void ImPlot::PlotBarGroups<signed char>(...)'

This PR makes the demo gate those snippets at compile time. The type list (IMPLOT_CUSTOM_NUMERIC_TYPES if defined, otherwise all ten types) is defined once as IMPLOT_NUMERIC_TYPES in implot_internal.h, moved there from implot_items.cpp. implot_demo.cpp gets a small "Numeric Type Gating" section that reads it, and an IMPLOT_DEMO_TYPE_GATED(Name) macro. It declares Name<T>() as two SFINAE overloads: the real body when T is enabled, and a one-line notice when it is not. The disabled overload never references PlotXxx<T>, so the linker never looks for it. The affected demos are Bar Plots, Bar Groups, Bar Stacks, the second Pie Chart, Markers and Text, Colorful Bars (Per-Index Colors), Linked Axes and the Seaborn style (Custom Styles). The list is also documented next to its definition, noting that the define must reach implot_demo.cpp too and that the demo needs float and double in it. A new Linux CI job builds and runs example_implot with (float)(double), so a demo that needs another type without being gated now fails to link there. With the default type list nothing changes: each demo calls the same code with the same type as before.

Before:

void Demo_BarPlots() {
    IMGUI_DEMO_MARKER("Plots/Bar Plots");
    static ImS8 data[10] = {1,2,3,4,5,6,7,8,9,10};
    ...
}

After:

IMPLOT_DEMO_TYPE_GATED(Demo_BarPlots_) {
    static T data[10] = {1,2,3,4,5,6,7,8,9,10};
    ...
}

void Demo_BarPlots() {
    IMGUI_DEMO_MARKER("Plots/Bar Plots");
    Demo_BarPlots_<ImS8>();
}

When the type is excluded, the demo shows:

void ShowTypeDisabledNotice() {
    ImGui::TextDisabled("(this demo needs a numeric type excluded via IMPLOT_CUSTOM_NUMERIC_TYPES)");
}

Tested by building example/ against ImGui v1.93.0 with (float)(double) (fails to link before this change, links after) and with the default types (unchanged).

Bar Plots, the second Pie Chart and Linked Axes, in the default build and with (float)(double):

default types	(float)(double)