Use texture LOD level 0 when drawing skybox

Rendering/OpenGL2/vtkOpenGLSkybox.cxx

@@ -143,7 +143,7 @@ void vtkOpenGLSkybox::Render(vtkRenderer* ren, vtkMapper* mapper)
         "  vec3 dirv = vec3(dot(diri,floorRight),\n"
         "    dot(diri,floorPlane.xyz),\n"
         "    dot(diri,floorFront));\n"
-        "  vec4 color = texture(actortexture, dirv);\n"
+        "  vec4 color = textureLod(actortexture, dirv, 0.0);\n"
         "//VTK::Gamma::Impl\n");
     }
     if (this->Projection == vtkSkybox::Sphere)
@@ -162,7 +162,7 @@ void vtkOpenGLSkybox::Render(vtkRenderer* ren, vtkMapper* mapper)
         "    dot(diri,floorFront));\n"
         "  float phix = length(vec2(dirv.x, dirv.z));\n"
         "  vec4 color = textureLod(actortexture, vec2(0.5*atan(dirv.x, "
-        "dirv.z)/3.1415927 + 0.5, atan(dirv.y,phix)/3.1415927 + 0.5), 1.0);\n"
+        "dirv.z)/3.1415927 + 0.5, atan(dirv.y,phix)/3.1415927 + 0.5), 0.0);\n"
         "//VTK::Gamma::Impl\n");
     }
     if (this->Projection == vtkSkybox::StereoSphere)
@@ -182,7 +182,7 @@ void vtkOpenGLSkybox::Render(vtkRenderer* ren, vtkMapper* mapper)
         "    dot(diri,floorFront));\n"
         "  float phix = length(vec2(dirv.x, dirv.z));\n"
         "  vec4 color = textureLod(actortexture, vec2(0.5*atan(dirv.x, dirv.z)/3.1415927 + "
-        "0.5, 0.5*atan(dirv.y,phix)/3.1415927 + 0.25 + 0.5*leftEye), 1.0);\n"
+        "0.5, 0.5*atan(dirv.y,phix)/3.1415927 + 0.25 + 0.5*leftEye), 0.0);\n"
         "//VTK::Gamma::Impl\n");
     }
     if (this->Projection == vtkSkybox::Floor)