Fixed bug #5756, Rendering artifacts when MRM is not set to maximum
Several changes were made:
* Added function multires_level_n to get the nth level from a multires mesh
* Removed the changes I made some time ago to init_render_mesh for multires meshes. Previously it was making a full copy of the mesh object in order to be able to apply deformations to the Pin level and propagate them to the Render level.
* Added two functions to DerivedMesh.c, multires_render_pin and multires_render_final. These two functions work together in the mesh_create_derived_*_render functions to apply all modifiers to the Pin level, then create the DerivedMesh from the Render level, and lastly restore the mesh to its original (undeformed) state.
* Added a check in multires_del_lower and multires_del_higher to ensure that level indices are properly clipped to the actual range of available levels.
a9cd5b808c