use native getComputedStyle on IE9 and IE10

this is a robustness enhancement as discussed on https://github.com/mootools/mootools-core/pull/2414#r1478811

This relegates the use of non-standard `currentStyle` to oldIE and Opera (which has a bug where sub-pixel values get rounded on native `getComputedStyle`, which can create jerky animations)

As you see this also finally kills the usage of browser.* sniffs on `getStyle`