Metal and Vulkan don't support line smoothing. There is a workaround
implemented. This workaround is only enabled when linesmooth value is
larger than 1. However When using smooth lines it should also be used.
This is fixed by adding a `GPU_line_smooth_get` function for getting the
current line smooth state.
Pull Request: https://projects.blender.org/blender/blender/pulls/138123
d4986be082