Do not draw inactive math corners when they have mathbg color

Do not draw inactive math corners when they have mathbg color

By default, inactive math corners are invisible. In practice they are
annoying because they are visible when selecting text, and they can
also overwrite some parts of the equation.

The code in Inset::drawMarkers2, which is only used for maths, is
moved to InsetMathHull. Moreover, the inactive corners are not drawn
when they have the same color as the math background. A better way to
achieve this would be to set the color to transparent, but we do not
support this yet.