fix for offset not respected (plenty everywhere)

-> this one should be fixed in lime as well 
-> it denotes low level code not well tested and freaks me out (for a change).
-> fact io.bytes are an array at source actually perform a rewrite and is a *inacceptable* bottleneck. You should fix it very quick.

use case : 
var pix = new Uint8Array(pixels.bytes.getData(), pixels.offset);
gl.texImage2D(GL.TEXTURE_2D, mipLevel, GL.RGBA, t.width, t.height, 0, GL.RGBA, GL.UNSIGNED_BYTE, pix);