Fixed read buffer and draw buffers

Specifying a read buffer or draw buffers was set on the context
instead of being set on the Framebuffer object, which led to
multiple Framebuffer objects wrongly sharing this state.

Change-Id: I9d137c52f50e58a48b95f7ed9a44b4dad275e710
Reviewed-on: https://swiftshader-review.googlesource.com/4745
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>