Fixed an error message when unlocking texture in spherical harmonics.

On some tracks it was reporting following message:
[warn   ] GLWrap: OpenGL debug callback - API
[warn   ] GLWrap:     Error type : ERROR
[warn   ] GLWrap:     Severity : HIGH
[warn   ] GLWrap:     Message : GL_INVALID_VALUE error generated. Size and/or offset out of range.

This fixes #2504

src/graphics/skybox.cpp

@@ -299,6 +299,7 @@ void Skybox::generateSpecularCubemap()
     }
     glBindFramebuffer(GL_FRAMEBUFFER, 0);
     glDeleteFramebuffers(1, &fbo);
+    glActiveTexture(GL_TEXTURE0);
 }   // generateSpecularCubemap