[gles] create_bind_group fails for buffers with MAP_WRITE

[Wumpf]

Description
Gles backend crashes when creating a bind group for a freshly created buffer that has MAP_WRITE usage.

Only got a vague wasm callstack, but pretty sure it's this unwrap here https://github.com/gfx-rs/wgpu/blob/v0.13/wgpu-hal/src/gles/device.rs#L903 failing because when going this early out path we don't have a raw buffer just yet https://github.com/gfx-rs/wgpu/blob/v0.13/wgpu-hal/src/gles/device.rs#L364

Repro steps

• create buffer with wgpu::BufferUsages::MAP_WRITE and wgpu::BufferUsages::UNIFORM
• create simple bindgroup like this

        let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
            label: None,
            layout: &bind_group_layout,
            entries: &[wgpu::BindGroupEntry {
                binding: 0,
                resource: uniform_buffer.as_entire_binding(),
            }],
        });

observe crash on unwrap.

Platform
Tested with webgl on the web

[Imberflur]

This may be related to some missing validation that I recently fixed: #3023.

Part of what this validation ensures is that MAP_WRITE can only be paired with COPY_SRC unless wgpu::Features::MAPPABLE_PRIMARY_BUFFERS is enabled.

Although MAPPABLE_PRIMARY_BUFFERS documentation claims it should be supported on all native platforms, but the gles and dx11 backends don't add it to the list of supported features so I'm not sure if that is true.

[Imberflur]

From #wgpu-users:matrix.org discussion it seems that MAPPABLE_PRIMARY_BUFFERS isn't supported on dx11 and gles and the docs are out of date.

[Imberflur]

In 0.14 this should now emit a validation error when trying to create the buffer.