Added setRenderTargets method to the RenderSupport

As the new Standard profile added the possibility to set multiple render targets I think it would be nice to have a way to acces this functionality from RenderSupport. While MRTs aren't really used by Starling in any way, I think it may prove handy to extension developers since everything related to RTs would be accessible through RenderSupport without mixed calls either to Context3D instance or RenderSupport depending on which version of setRenderTarget you need.

I've also added an optional parameter for enabling depth and stencil buffers if rendering to a texture.