stdweb's read_pixels is impossible to use

[kvark]

Looks like it expects AsArrayBufferView which works with immutable slices. This doesn't make sense for read_pixels, because we expect the slice to be filled up. Erasing the mutability from it looks like a giant UB to me:

    --> src/stdweb.rs:2695:69
     |
2695 |                 gl.read_pixels(x, y, width, height, format, gltype, Some(data))
     |                                                                     ^^^^^^^^^^ the trait `webgl_stdweb::AsArrayBufferView<'_>` is not implemented for `&mut [u8]`
     |
     = help: the following implementations were found:
               <&'a [f32] as webgl_stdweb::AsArrayBufferView<'a>>
               <&'a [f64] as webgl_stdweb::AsArrayBufferView<'a>>
               <&'a [i16] as webgl_stdweb::AsArrayBufferView<'a>>
               <&'a [i32] as webgl_stdweb::AsArrayBufferView<'a>>
             and 4 others
     = note: `webgl_stdweb::AsArrayBufferView<'_>` is implemented for `&[u8]`, but not for `&mut [u8]`