Add support for multisampled rendertargets in most OpenGL platforms

Tested on WindowsGL, iOS (iPad mini, air and iPhone 5s) and Android (Nexus5, Nexus7, Samsung Galaxy Tab 2 7inch, Samsung Galaxy Tab 4 8inch).

This PR also pave the way for MRT and RenderTargetCube support on those platforms (when available) but I decided to keep this as simple as possible so it can be easily merged.

There are a couple more extensions available to extend support to more Android devices but the ones in this PR are the most common (and robust).