Improve explanation/figures for italic correction in sub/sup scripts

Created by: fred-wang

Follow-up of #28 (closed) by @jahewson

• Figure 20 uses LargeOpItalicCorrection but Figure 21 uses ItalicCorrection, while Figure 22 uses neither but the text says if largeop="true" then italic correction actually is used?
• I find it quite difficult to see the difference between the red and orange colors, can the contrast between them be increased?

See https://github.com/MathML/MathMLinHTML5/issues/28#issuecomment-224823276 I think the best will be to move the italic corrections in separate figures, showing the cases where the base is a largeop or not.