Shadows should cast onto water surface

[past-due]

At the moment, the cascaded shadow maps are not used for rendering water (i.e. shadows only cast on the terrain floor underneath the water).

The water shaders (terrain_water_high.vert, terrain_water_high.frag) will require modifications to support this.

Notes:

• WZ will need to pass in various shadow-map related uniforms and the shadowMap texture to the high terrain water shader
• The shader will need to apply similar logic as terrain_combined_high.frag to use the shadow-related functions in terrain_combined_frag.glsl