Improve shadows

[Philip Taylor]

Shadows could do with significant improvements. #504 and #661 and #143 are somewhat relevant. Some other random thoughts:

• GL_LINEAR filtering on depth textures shouldn't really work. In particular it doesn't work at all on ATI, so it looks ugly. NVIDIA does something funny (PCF?). Intel maybe does something like NVIDIA. We ought to have it work properly with GL_NEAREST so it's not ugly on ATI.
• Some kind of ​CSM might be nice.
• Users should be able to configure higher-res shadow maps. Maybe we should default to higher-res if it doesn't hurt performance much?
• We can drop non-FBO and non-depth-texture paths.