Default drawBuffer/readBuffer for non default Framebuffer

Default drawBuffer/readBuffer for non default Framebuffer

drawBuffer and readBuffer default to GL_BACK in the default
Framebuffer, but for all other Framebuffer objects, the
default value should have been GL_COLOR_ATTACHMENT0.

Fixes all failures in dEQP-GLES3.functional.state_query.integers*

Change-Id: I060db67c50561d2678d57de1d7067bd7f8d2a53a
Reviewed-on: https://swiftshader-review.googlesource.com/14948
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>