Fix #74, by forcing scroll views to scroll up until items are visible.

[DaleStan]

This fixes at least the object explorer (SelectObjectPanels) and the production sheet list (MainScreen.allPages), and doesn't visibly affect the production tables.

[shpaass]

Thank you for the fix! I tested it, and it works.

Could you please document the purpose of the fields firstVisibleBlock and bufferRows? They are not as easy to understand as the other ones.

Since the class structure of the GUI is complicated, I would also appreciate a comment here about which call of BuildContents() this PR fixes.

[DaleStan]

The call is guiBuilder(this) at ImGuiBuilding.cs:173. The fixed delegate that's called there is created by contents = new ImGui(BuildContents, padding, clip: true); in the ScrollAreaBase constructor, when constructing a VirtualScrollList<> or one of its derived classes.

If you're asking how I found that code to fix, I started by discovering that maxScroll had the old value in scroll2d, and then that maxScroll wasn't being updated because MeasureContent was returning the old value. That was because contentSize was wrong, because lastContentRect was wrong. I think that got me close enough to DoGui and guiBuilder(this) so I could step into them and decide that it didn't make sense to have firstRow >= rowCount. (There were, of course, a lot of wrong turns too.)

[shpaass]

Thank you so much for the walkthrough!