dont clear empty FBOs, fix doc example

apps/openmw/mwrender/postprocessor.cpp

@@ -662,6 +662,11 @@ namespace MWRender
 
                 for (const auto& name : pass->getRenderTargets())
                 {
+                    if (name.empty())
+                    {
+                        continue;
+                    }
+
                     auto& renderTarget = technique->getRenderTargetsMap()[name];
                     subPass.mStateSet->setTextureAttribute(subTexUnit, renderTarget.mTarget);
                     subPass.mStateSet->addUniform(new osg::Uniform(name.c_str(), subTexUnit));

docs/source/reference/postprocessing/omwfx.rst

@@ -561,7 +561,7 @@ color buffer will accumulate.
         source_format = rgb;
         internal_format = rgb16f;
         source_type = float;
-        clear_color = vec4(1,0,0,1);
+        clear_color = vec4(0,0,0,1);
     }
 
     fragment red(target=RT_Red,blend=(add, src_color, one), rt1=RT_Red) {