Fix crash on passing empty span to ImDrawList::AddText

[ZingBallyhoo]

Context: in the current version, passing an empty span to ImDrawList::AddText will cause crash in Encoding. The manually written methods were also not fixed for #436

• Added generator support for methods that take char* text_start, char* text_end, removed manually written versions (igCalcTextSize & ImDrawList::AddText). This also results in more overloads being available for them.
• Fixed crashes in Util.CalcSizeInUtf8 & GetUtf8 when passing empty spans
• Switched to Util.GetUtf8 for marshalling string arrays
• Regenerated ImPlot&ImNodes as they have not been recently updated

[zaafar]

i will try to get this merge this weekend.