Minor improvements to child window resizing behavior

[cfillion]

Child window resizing is currently clamped to the parent window only if the child window itself has no scrollbar on that axis. It seems unintuitive to look at the child window's scrollbars rather than at the parent window's.

In a situation where the parent window is non-resizable, has scrollbars+mousewheel disabled and persistent storage enabled, that makes it very easy for end users to accidentally brick their UI until the .ini is manually cleared.

This PR also changes the clamp rect to take into account window padding. Currently it has half the window padding at the right side and no padding at the bottom. It's asymmetrical and makes it hard to neatly align a resizable child with other full-width items (eg. -FLT_MIN).

Lastly, this PR extends child window resize grips to cover the entire border length since childs don't have diagonal resize grips at the corners.

[ocornut]

Hello,
I've merged two out of three commits.
Looking at parent flags seems correct, amends fa4c49b.

Using WorkRect instead of InnerClipRect is however incorrect. WorkRect encompass the scrolling area.
I suppose you'd want to use InnerRect + apply WindowPadding over it:

ImRect border_limit_rect = window->ParentWindow->InnerRect;
border_limit_rect.Expand(ImVec2(-ImMax(window->WindowPadding.x, window->WindowBorderSize), -ImMax(window->WindowPadding.y, window->WindowBorderSize)));
if ((parent_flags & (ImGuiWindowFlags_HorizontalScrollbar | ImGuiWindowFlags_AlwaysHorizontalScrollbar)) == 0 || (parent_flags & ImGuiWindowFlags_NoScrollbar))
    border_target.x = ImClamp(border_target.x, border_limit_rect.Min.x, border_limit_rect.Max.x);
if (parent_flags & ImGuiWindowFlags_NoScrollbar)
    border_target.y = ImClamp(border_target.y, border_limit_rect.Min.y, border_limit_rect.Max.y);

I pushed that as 742e534

Eventually we should apply #3312 and might add a helper rectangle corresponding to that.

Thank you!