Merge branch 'SampleSDL2' of https://github.com/CedricGuillemet/imgui into CedricGuillemet-SampleSDL2

Fixed ListBoxHeader() not honoring negative sizes the same way as BeginChild() or BeginChildFrame() (#263)

May trigger new compilation warnings?

ImDrawList store pointer to their owner name for easier auditing/debugging. Metrics windows render clip rectangle when hovering.

PlotLines(), PlotHistogram(): fixed incorrect hovering test (would trigger tooltip when menu is open)