// - Use child windows to begin into a self-contained independent scrolling/clipping regions within a host window. Child windows can embed their own child.
// - For each independent axis of 'size': ==0.0f: use remaining host window size / >0.0f: fixed size / <0.0f: use remaining window size minus abs(size) / Each axis can use a different mode, e.g. ImVec2(0,400).
// - BeginChild() returns false to indicate the window is collapsed or fully clipped, so you may early out and omit submitting anything to the window.
// Always call a matching EndChild() for each BeginChild() call, regardless of its return value.
// [Important: due to legacy reason, this is inconsistent with most other functions such as BeginMenu/EndMenu,
// BeginPopup/EndPopup, etc. where the EndXXX call should only be called if the corresponding BeginXXX function
// returned true. Begin and BeginChild are the only odd ones out. Will be fixed in a future update.]
// - BeginChild() returns false to indicate the window is collapsed or fully clipped, so you may early out and omit submitting
// anything to the window. Always call a matching EndChild() for each BeginChild() call, regardless of its return value.
// [Important: due to legacy reason, Begin/End and BeginChild/EndChild are inconsistent with all other functions
// such as BeginMenu/EndMenu, BeginPopup/EndPopup, etc. where the EndXXX call should only be called if the corresponding
// BeginXXX function returned true. Begin and BeginChild are the only odd ones out. Will be fixed in a future update.]