Handle WGPUVertexStepMode_Undefined#490

AlbinBernhardssonARM wants to merge 1 commit intogfx-rs:trunkgfx-rs/wgpu-native:trunkfrom

AlbinBernhardssonARM:WGPUVertexStepMode_UndefinedAlbinBernhardssonARM/wgpu-native:WGPUVertexStepMode_UndefinedCopy head branch name to clipboard