Fix textureGather compatibility on macOS 10.13

[xiaopengli89]

thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
    Internal error in VERTEX shader: Metal: Compilation failed: 

program_source:90:65: error: use of undeclared identifier 'int2'; did you mean 'int'?
            metal::float4 _e4 = image.gather(samp, tex_coord_1, int2(0), metal::component::y);
                                                                ^
program_source:94:65: error: use of undeclared identifier 'int2'; did you mean 'int'?
            metal::float4 _e5 = image.gather(samp, tex_coord_1, int2(0), metal::component::z);
                                                                ^
program_source:98:65: error: use of undeclared identifier 'int2'; did you mean 'int'?
            metal::float4 _e6 = image.gather(samp, tex_coord_1, int2(0), metal::component::w);
                                                                ^
program_source:196:98: warning: suggest braces around initialization of subobject
                fitting_lanczos_weight(weight_color_1, weight_sum_1, _e74, point_ori_1, type_10 {metal::float2(gather_center.x - 0.5, gather_center.y + 0.5), metal::float2(gather_center.x + 0.5, gather_center.y + 0.5), metal::float2(gather_center.x + 0.5, gather_center.y - 0.5), metal::float2(gather_center.x - 0.5, gather_center.y - 0.5)}, fitting_scale);
                                                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                                                 {                                                                                                                                                                                                                                                 }


', /Users/admin/.cargo/git/checkouts/wgpu-53e70f8674b08dd4/754a4ba/wgpu/src/backend/[direct.rs:2418](http://direct.rs:2418/):5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[teoxoy]

Thanks for this! Just a small improvement and it should be good to go!