This avoids recompiles for shaders that don't use explicit derivatives
when ctx->Hint.FragmentShaderDerivative == GL_NICEST.
For example, GFXBench 5 Aztec Ruins sets the GL_NICEST hint before
compiling any shaders, but none of them use dFdx() or dFdy() - only
implicit derivatives. This doesn't eliminate any recompiles, but
does eliminate one of the reasons for doing so.
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
992e2cf57f