Retina support

[drewish]

Curious if there's recommendations for the best way to get retina support working. Reading through the ImGui issue queue it sounds like they suggest just doubling the font sizes. You can't specify a size for the default font so it seems like you need to manually load a font (that's what prompted #40 asking about loading fonts by name).

This seems to work reasonably but I'd love to stick with the default font:

ui::initialize( ui::Options().font( "/Library/Fonts/Arial.ttf", 13.0 * getWindow()->getContentScale() ) );

I opened up ocornut/imgui#1076 to see about getting a parameter added to specify a font size for the default font.