mesa: don't always enable OES_standard_derivatives

For Intel, expose it only if gen >= 4.
For Gallium, expose it only if PIPE_CAP_SM3 is advertised.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/standalone_scaffolding.cpp

@@ -80,6 +80,7 @@ void initialize_context_to_defaults(struct gl_context *ctx, gl_api api)
    ctx->Extensions.EXT_texture3D = true;
    ctx->Extensions.OES_EGL_image_external = true;
    ctx->Extensions.ARB_shader_bit_encoding = true;
+   ctx->Extensions.OES_standard_derivatives = true;
 
    ctx->Const.GLSLVersion = 120;