Shield detail settings / rendering cleanup

Shields build temporary meshes to render impact effects on`(code/ship/shield.cpp create_shield_explosion())`. On max detail level these are not used because decal based rendering and the raw shield mesh are used`(code/ship/shield.cpp render_shield(int))` . We should not build the mesh at this detail setting, and we should probably consider going fully to decal based rendering for hits.

I have a prototype branch of the first path that I'm currently testing.