Apllied shadows to resources' textures

[stavrosfa]

Basically what the title says.

An alternative way of doing this that I tested, instead of exposing a public getter for ModernGraphicsActive and checking it in the ResourceLayer, would be to add an empty texture so Lua doesn't freak out, and render a cropped region of nothing (since newer graphics can simply contain their shadows in the same image) when the new graphics are enabled, which I didn't like, it's just wasted draw calls.

Maybe a boolean in Lua like "Civ3Exclusive" would be helpful, but then the Texture.Load() would have to return null or something that doesn't break everything and can be checked by the caller merhod in order to skip the draw call.

Let me know what you think.