UI exception on selected text in textbox

[ColinPitrat]

When selecting text across lines in multiline text in a textbox and unselecting from the right, an exception is raised when reaching the beginning of the line.

At the time of writing, this happens in:

#4  0x0000555555660e82 in wGui::CTextBox::Draw (this=0x55555613bc00) at src/gui/src/wg_textbox.cpp:242          
242                                                     SelRect.SetRight(vCharRects.at(CurLine).at(SelEndPoint.XPos() - 1).Right() +    

This code is wrong because it should handle selection at the char level. On the other hand, it's probably necessary to keep coordinates of selection for movements up and down.

[ColinPitrat]

Note: this is not a big deal because unless I'm mistaken, we don't use an editable textbox anywhere in Caprice.