Added support for EXT versions of OpenGL framebuffer object (FBO) method...

This patch adds support for the EXT versions of the OpenGL framebuffer object (FBO) methods. The EXT versions will only be used if the ARB versions are not supported. It will also check for FBO support and throw an exception if it's not supported.

This should address issues reported in issue #998 and also issue #811.

I tested this under Windows 7 on my laptop (which only has the EXT FBO methods) and also my desktop (which supports the ARB versions). This should also work under Linux and MacOS, but I was unable to test this. I also had to make changes to all other projects using OpenGL to support this patch, but I was also unable to test this. Hopefully I haven't missed anything and everything is still working as expected.

PS. This is my first every pull request, so please be gentle :)