Try to fix #1772

Avoid doing glViewport call in IBL.cpp since the result is not used
without dynamic light.

src/graphics/IBL.cpp

@@ -260,6 +260,9 @@ GLuint generateSpecularCubemap(GLuint probe)
         glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, GL_RGBA16F, cubemap_size, cubemap_size, 0, GL_BGRA, GL_FLOAT, 0);
     glGenerateMipmap(GL_TEXTURE_CUBE_MAP);
 
+    if (!UserConfigParams::m_dynamic_lights)
+        return cubemap_texture;
+
     GLuint fbo;
     glGenFramebuffers(1, &fbo);
     glBindFramebuffer(GL_FRAMEBUFFER, fbo);