Fix #122047: Correct text selection when text is scrolled

Correct calculation of start of visual string and position of the
selection start when the text is scrolled (showing a subset of the
text) within an input box.

Pull Request: https://projects.blender.org/blender/blender/pulls/122110

source/blender/editors/interface/interface_widgets.cc

@@ -1946,9 +1946,9 @@ static void widget_draw_text(const uiFontStyle *fstyle,
       immBindBuiltinProgram(GPU_SHADER_3D_UNIFORM_COLOR);
       immUniformColor4ubv(wcol->item);
       const auto boxes = BLF_str_selection_boxes(fstyle->uifont_id,
-                                                 drawstr,
+                                                 drawstr + but->ofs,
                                                  strlen(drawstr),
-                                                 but->ofs + but->selsta,
+                                                 but->selsta - but->ofs,
                                                  but->selend - but->selsta);
       for (auto bounds : boxes) {
         immRecti(pos,