Fix #112642: Anchor Word Start For Double-click Word Select

In Interface text inputs, when double-clicking to select a word, set
the "initial selection position" to the beginning of the word. This way
you can continue to select forward by dragging before releasing mouse.

Pull Request: https://projects.blender.org/blender/blender/pulls/113037

source/blender/editors/interface/interface_handlers.cc

@@ -3803,6 +3803,7 @@ static void ui_do_but_textedit(
         but->pos = short(selend);
         but->selsta = short(selsta);
         but->selend = short(selend);
+        data->sel_pos_init = selsta;
         retval = WM_UI_HANDLER_BREAK;
         changed = true;
       }