checkptr failure

[dertseha]

Found through inkyblackness/imgui-go#98 , running an example with go run -race -tags 'sdl' ., the race detector will fail at the call to gl.VertexAttribPointer(..., unsafe.Pointer(...)) .

This is due to the wrong signature of gl.VertexAttribPointer(), which should actually use uintptr as the last parameter.

This needs a fix in the lower library, tracked here: go-gl/gl#124

[dertseha]

One year later, this is done.