Custom per-item colors for line and scatter plot

[mihaly-sisak]

Hello!
I've implemented user-settable per-item colors for line and scatter plots.
I've tried to match your coding style as close as I was able.
Please consider merging this pull request, or provide feedback how can I improve the implementation so it can be merged.
All previously available public plot APIs are unchanged.
I've added two new plot types:

typedef ImU32 (*ImPlotColorGetter)(ImPlotCol col, int idx, void* user_data);
IMPLOT_API void PlotLineCG(const char* label_id, ImPlotGetter getter_func, void* data, int count, ImPlotColorGetter color_func, void* color_data, ImPlotLineFlags flags=0);
IMPLOT_API void PlotScatterCG(const char* label_id, ImPlotGetter getter, void* data, int count, ImPlotColorGetter color_func, void* color_data, ImPlotScatterFlags flags=0);

The user can write a callback to set the item color based on the index.
I've added to the demo to showcase the new APIs.
The single-color plots are drawn with a new struct ColorGetter that is basically just a wrapper around ImGui::GetColorU32(GetItemData().Colors[col]).
I wanted to cache the plot colors so the ImVec4->ImU32 conversion happens only once. If you can give me any direction how can I achieve this more elegantly (than the current fill_colors()) please point me to the right direction. With this setup the ImVec4->ImU32 conversion happens only once, just as before with the single-color only version.
The per-item-colored plots are drawn with struct ColorGetterFuncPtr, implemented in the same style as struct GetterXY and struct GetterFuncPtr.