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.
Added global Alpha in ImGuiStyle + commented ImGuiStyle fields in .h
Initialised window->Accessed in constructor. Begin() return false with Alpha==0.0
Undo Begin() return false with Alpha==0.0, misleading at the moment
Skip rendering if alpha is 0.0
Skip most logic is alpha is 0.0, Begin() also return false to allow user to early out
Removing line from Todo list
Fixed tooltip size (broken earlier today) + added todo items
Removed unused parameter in demo window code
Todo list
Converted all Tabs to Spaces (git diff -w shows an empty diff)
Added an assertion
Exposed CalcTextSize(), GetCursorScreenPos() for more advanced fiddling
Converted all Tabs to Spaces

Argh