TODO [ENGINE][GFX]: Add "alpha to coverage" flag for objects from scene file

Some RenderingObjects (i.e., trees) might need "alpha-to-coverage" blend state (otherwise, Gbuffer has useless alpha geometry, GI is breaking a bit, etc). A possibility to set and read this flag from the scene's file should be added. Currently, only the foliage system has "alpha-to-coverage" by default. 