Minor todo/readme changes
Fix clipping of title bar text.
Allowing the user to call NewFrame() multiple times without calling Render()

Note that this is never a good idea. But, allowing it reduce confusion
in the initial stage of setup.
Reduce inner window clipping to take account for the extend of CollapsingHeader

from arikwestbrook
Child window with inverted clip rectangles are marked as collapsed.
Renamed ImVector<> members.
Invisible child windows gets clipped earlier in the pipeline.
Added InputFloat4(), SliderFloat4() helpers.
memory override attempt #2
removed libimgui.pro