Make edge overdraw configurable beyond blending.

[darionco]

A problem that occurs with simple blending is that when there is a lot of overdraw the alpha values of the edges need to be lowered too much, this makes the parts with overdraw stand out but the rest of the edges basically disappear. This can create a visually pleasant effect (like smoke) in some areas but hides edge information.

My proposed solution is to take advantage of the double pass rendering implemented for RenderingMode.HIGH and use the first pass to render to a single channel data texture with pure additive blending (GL_ONE, GL_ONE) this should create an overdraw texture. The second pass uses this overdraw texture to render the edges in a color selected form a configurable color palette, effectively making the render result of overdraw configurable.

The texture can be cached for use in the other RenderModes so depth testing can remail enabled, ovedraw is one of the biggest performance hits when rendering large number of edges, so this would also improve performance.