More blitFramebuffer fixes

More blitFramebuffer fixes

A few minor adjustments:
- The validity of blit operations with the same bounds on source
  and destination buffer also extends to color buffers
- Renderbuffers, whether GL_RENDERBUFFER or GL_FRAMEBUFFER_DEFAULT
  are considered of the same type for the purpose of blitting

Change-Id: I0de7c19aa8b07fa57ed33b98be4a4ab609e6e115
Reviewed-on: https://swiftshader-review.googlesource.com/8213
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>