Eevee: Fix generated coordinates when no texture coordinates connected

Orco should behave the same if it comes from unconnected vec inputs, or
from the Texture Coordinate -> Generated node output.

Fixup for 11e7e0769a.

This is related to T52528. The coordinates are still different between
Eevee and Cycles, but at least it behaves consistent within itself.

In fact the shader should now be correct, but the orco attributes we are
passing the shader seems to be where the problem is. But it's to be
tackled separately.

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -2472,7 +2472,7 @@ void generated_from_orco(vec3 orco, out vec3 generated)
 #ifdef VOLUMETRICS
 	generated = worldPosition;
 #else
-	generated = orco;
+	generated = orco * 0.5 + 0.5;
 #endif
 }