Improved ImGuiListClipper helper

[ocornut]

Many times using the clipper helper I thought that it needed to be improved. Since the requirement of passing item height manually is rather error-prone.

Also issues e.g. #660

Currently typically use is:

ImGuiListClipper clipper(lines, ImGui::GetTextLineHeightWithSpacing());
for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
    ImGui::Text("%i The quick brown fox jumps over the lazy dog\n", i);
clipper.End();

And improved version could be:

ImGuiListClipper clipper(lines);
while (clipper.Step())
    for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
        ImGui::Text("%i The quick brown fox jumps over the lazy dog\n", i);

(unsure about Step function name - any suggestion?)

The idea is that:

• The first time Step() function is called would return true, set DisplayStart to 0 and DisplayEnd to 1
• This first loop would be used to calculate the item height
• The second time Step() function is called it would return true, set DisplayStart/DisplayEnd to actual values based on clipping.
• The third time Step() function is called it would return false and call "End" which is the final positional seeking.

Interesting this improvement seems possible without breaking the current code.
I would make the second parameter of the construction -1 by default.

[ocornut]

Done.
7a28f5b
28b0919