[ImGui] Allow setting of custom font config to enable merging of multiple fonts and other features

[rh101]

Describe your changes

When adding multiple fonts to ImGui, the only way to merge fonts is to set the ImFontConfig::MergeMode = true, otherwise glyphs from additional fonts do not render correctly.

This change allows custom ImFontConfig to be set on a font that is added via ImGuiPresenter::addFont. This change does not affect existing usage of the ImGuiPresenter::addFont method, because the fontConfig parameter has a default value of ImFontConfig(). The default value is valid, since it is what ImGui does internally if a nullptr is passed to one of the ImGui::AddFontXYZ methods.

For example:

auto* presenter = extension::ImGuiPresenter::getInstance();
presenter->addFont("fonts/arial.ttf", 14.f); // This will use the default ImFontConfig

ImFontConfig fontConfig;
fontConfig.MergeMode = true; // Set to true for any additional fonts if merging is required. This must be false for the first font in.
presenter->addFont("fonts/fa-regular-400.ttf", 14.f, { ICON_MIN_FA, ICON_MAX_FA, 0 } , fontConfig); // example of adding Font Awesome glyphs

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