Implement GL_EXT_texture_shadow_lod

[Ralith]

GL_EXT_texture_shadow_lod provides various missing overloads for sampling sampler2DArrayShadows. Their absence in GLSL is baffling and an awkward constraint in e.g. cascading shadow map implementations.

In theory you can work around this with textureGrad, but this seems to have significantly worse performance than technically-UB use of texture on my 970, accounting for more than 1ms of frametime.