Fix alpha layers when using with scrolling.

Take the scroll position into account to draw the layer at the right
place.
Enable 3D_RENDERING and TEXTURE_MAPPER

This gets things using 3D transform to render, even if the transforms
are not completely used (they are reduced to 2D affine transforms instead)

Since many websites use an identity transform as a way to force GPU
rendering on many browsers, this fixes at least those:
 * Search bar on youtube
 * 2048 game (now play complete with tile animations)
 * Sberbank.ru login page
And possibly more.